Make std::filesystem::path conversion to/from UTF-8 encoded string explicit

[risa2000]

This PR makes the default conversions of std::filesystem::path from/to JSON UTF-8 encoded string explicit.
This is a follow up on #4271.

Original implementation relied on the platform locale to be UTF-8 and used std::string as std::filesystem::path conversion source/target. This did not work on Windows (where the implicit locale is using "multibyte" encoding) and may potentially trigger a similar issue on platforms which do not have the locale set to UTF-8.

Proposed change uses explicit conversion functions from std library to UTF-8 encoded string. Unfortunately, the UTF-8 handling of std library changed from C++17 to C++20 so the code uses different codepaths for corresponding C++ standards.

This code has been inspired by:
https://github.com/alf-p-steinbach/C---how-to---make-non-English-text-work-in-Windows/blob/main/microlibs/cppm/stdlib_workarounds/fs_path.hpp#L19

[coveralls]

coverage: 99.186%. remained the same
when pulling 008c50b on risa2000:patch-fspath-conversions
into e90c860 on nlohmann:develop.

[risa2000]

I am bit confused about how make amalgamate works. It seems to arbitrary change formatting on some files, even when I do only a small change (e.g. unit-conversions.cpp) and sometimes it looks like those changes are different each time I run it?

I could not run it on Windows (seems like Makefile is not compatible with nmake) so I run it on macOS.

[nlohmann]

I am bit confused about how make amalgamate works. It seems to arbitrary change formatting on some files, even when I do only a small change (e.g. unit-conversions.cpp) and sometimes it looks like those changes are different each time I run it?

asytle and the amalgamation is deterministic. There should not be different results after subsequent calls.

I could not run it on Windows (seems like Makefile is not compatible with nmake) so I run it on macOS.

It's based on Python. The calls the Makefile makes are:

python3 -mvenv tools/astyle/venv
tools/astyle/venv/bin/pip3 install --quiet --upgrade pip
tools/astyle/venv/bin/pip3 install --quiet -r tools/astyle/requirements.txt
tools/amalgamate/amalgamate.py -c tools/amalgamate/config_json_fwd.json -s . --verbose=yes
tools/astyle/venv/bin/astyle --project=tools/astyle/.astylerc include/nlohmann/adl_serializer.hpp include/nlohmann/byte_container_with_subtype.hpp include/nlohmann/detail/abi_macros.hpp include/nlohmann/detail/conversions/from_json.hpp include/nlohmann/detail/conversions/to_chars.hpp include/nlohmann/detail/conversions/to_json.hpp include/nlohmann/detail/exceptions.hpp include/nlohmann/detail/hash.hpp include/nlohmann/detail/input/binary_reader.hpp include/nlohmann/detail/input/input_adapters.hpp include/nlohmann/detail/input/json_sax.hpp include/nlohmann/detail/input/lexer.hpp include/nlohmann/detail/input/parser.hpp include/nlohmann/detail/input/position_t.hpp include/nlohmann/detail/iterators/internal_iterator.hpp include/nlohmann/detail/iterators/iter_impl.hpp include/nlohmann/detail/iterators/iteration_proxy.hpp include/nlohmann/detail/iterators/iterator_traits.hpp include/nlohmann/detail/iterators/json_reverse_iterator.hpp include/nlohmann/detail/iterators/primitive_iterator.hpp include/nlohmann/detail/json_custom_base_class.hpp include/nlohmann/detail/json_pointer.hpp include/nlohmann/detail/json_ref.hpp include/nlohmann/detail/macro_scope.hpp include/nlohmann/detail/macro_unscope.hpp include/nlohmann/detail/meta/call_std/begin.hpp include/nlohmann/detail/meta/call_std/end.hpp include/nlohmann/detail/meta/cpp_future.hpp include/nlohmann/detail/meta/detected.hpp include/nlohmann/detail/meta/identity_tag.hpp include/nlohmann/detail/meta/is_sax.hpp include/nlohmann/detail/meta/std_fs.hpp include/nlohmann/detail/meta/type_traits.hpp include/nlohmann/detail/meta/void_t.hpp include/nlohmann/detail/output/binary_writer.hpp include/nlohmann/detail/output/output_adapters.hpp include/nlohmann/detail/output/serializer.hpp include/nlohmann/detail/string_concat.hpp include/nlohmann/detail/string_escape.hpp include/nlohmann/detail/string_utils.hpp include/nlohmann/detail/value_t.hpp include/nlohmann/json.hpp include/nlohmann/json_fwd.hpp include/nlohmann/ordered_map.hpp include/nlohmann/thirdparty/hedley/hedley.hpp include/nlohmann/thirdparty/hedley/hedley_undef.hpp tests/abi/config/config.hpp tests/abi/config/custom.cpp tests/abi/config/default.cpp tests/abi/config/noversion.cpp tests/abi/diag/diag.cpp tests/abi/diag/diag.hpp tests/abi/diag/diag_off.cpp tests/abi/diag/diag_on.cpp tests/abi/inline_ns/use_current.cpp tests/abi/inline_ns/use_v3_10_5.cpp tests/abi/main.cpp tests/benchmarks/src/benchmarks.cpp tests/cmake_add_subdirectory/project/main.cpp tests/cmake_fetch_content/project/main.cpp tests/cmake_fetch_content2/project/main.cpp tests/cmake_import/project/main.cpp tests/cmake_import_minver/project/main.cpp tests/cmake_target_include_directories/project/Bar.cpp tests/cmake_target_include_directories/project/Bar.hpp tests/cmake_target_include_directories/project/Foo.cpp tests/cmake_target_include_directories/project/Foo.hpp tests/cmake_target_include_directories/project/main.cpp tests/cuda_example/json_cuda.cu tests/src/fuzzer-driver_afl.cpp tests/src/fuzzer-parse_bjdata.cpp tests/src/fuzzer-parse_bson.cpp tests/src/fuzzer-parse_cbor.cpp tests/src/fuzzer-parse_json.cpp tests/src/fuzzer-parse_msgpack.cpp tests/src/fuzzer-parse_ubjson.cpp tests/src/make_test_data_available.hpp tests/src/test_utils.hpp tests/src/unit-32bit.cpp tests/src/unit-algorithms.cpp tests/src/unit-allocator.cpp tests/src/unit-alt-string.cpp tests/src/unit-assert_macro.cpp tests/src/unit-binary_formats.cpp tests/src/unit-bjdata.cpp tests/src/unit-bson.cpp tests/src/unit-byte_container_with_subtype.cpp tests/src/unit-capacity.cpp tests/src/unit-cbor.cpp tests/src/unit-class_const_iterator.cpp tests/src/unit-class_iterator.cpp tests/src/unit-class_lexer.cpp tests/src/unit-class_parser.cpp tests/src/unit-class_parser_diagnostic_positions.cpp tests/src/unit-comparison.cpp tests/src/unit-concepts.cpp tests/src/unit-constructor1.cpp tests/src/unit-constructor2.cpp tests/src/unit-convenience.cpp tests/src/unit-conversions.cpp tests/src/unit-custom-base-class.cpp tests/src/unit-deserialization.cpp tests/src/unit-diagnostic-positions-only.cpp tests/src/unit-diagnostic-positions.cpp tests/src/unit-diagnostics.cpp tests/src/unit-disabled_exceptions.cpp tests/src/unit-element_access1.cpp tests/src/unit-element_access2.cpp tests/src/unit-hash.cpp tests/src/unit-inspection.cpp tests/src/unit-items.cpp tests/src/unit-iterators1.cpp tests/src/unit-iterators2.cpp tests/src/unit-iterators3.cpp tests/src/unit-json_patch.cpp tests/src/unit-json_pointer.cpp tests/src/unit-large_json.cpp tests/src/unit-locale-cpp.cpp tests/src/unit-merge_patch.cpp tests/src/unit-meta.cpp tests/src/unit-modifiers.cpp tests/src/unit-msgpack.cpp tests/src/unit-no-mem-leak-on-adl-serialize.cpp tests/src/unit-noexcept.cpp tests/src/unit-ordered_json.cpp tests/src/unit-ordered_map.cpp tests/src/unit-pointer_access.cpp tests/src/unit-readme.cpp tests/src/unit-reference_access.cpp tests/src/unit-regression1.cpp tests/src/unit-regression2.cpp tests/src/unit-serialization.cpp tests/src/unit-testsuites.cpp tests/src/unit-to_chars.cpp tests/src/unit-type_traits.cpp tests/src/unit-ubjson.cpp tests/src/unit-udl.cpp tests/src/unit-udt.cpp tests/src/unit-udt_macro.cpp tests/src/unit-unicode1.cpp tests/src/unit-unicode2.cpp tests/src/unit-unicode3.cpp tests/src/unit-unicode4.cpp tests/src/unit-unicode5.cpp tests/src/unit-user_defined_input.cpp tests/src/unit-windows_h.cpp tests/src/unit-wstring.cpp tests/src/unit.cpp single_include/nlohmann/json.hpp single_include/nlohmann/json_fwd.hpp docs/mkdocs/docs/examples/*.cpp

[risa2000]

After I pushed my PR, I noticed here when checking the diff that in my PR the amalgamation changed unit-conversions.cpp format in a way it obscured the change. So I run make amalgamate again and it seemed to change the formatting again, but still making changes on many more places. So I am giving up - I do not know how to keep the original format while running make amalgamate. Probably formatting changed meanwhile.