Add C++20 module build target

Usage in client cmake:
```cmake
set(FLECS_M0DULE ON CACHE BOOL "Build flecs module" FORCE)
add_subdirectory(path/to/flecs)
target_compile_features(flecs-module PUBLIC cxx_std_23)
target_link_libraries(
  someProjectName
  PRIVATE
  flecs-module
)
```

In the code, it is then possible to use `import flecs;` to import all the flecs symbols.

Still investigating why reflection seems to break, disabled in the module for now.

Maintainance:
- structs exposed by flecs must be tagged with `FLECS_API_STRUCT`
- code within the c++ addon tagged with `FLECS_API` are exposed
- globals must be tagged with `FLECS_API_GLOBAL`, which becomes `static` in headers, and export in the module.

CMakeLists.txt

@@ -7,6 +7,7 @@ option(FLECS_STATIC "Build static flecs lib" ON)
 option(FLECS_SHARED "Build shared flecs lib" ON)
 option(FLECS_PIC "Compile static flecs lib with position independent code (PIC)" ON)
 option(FLECS_TESTS "Build flecs tests" OFF)
+option(FLECS_M0DULE "Build flecs C++20 module" OFF)
 
 include(cmake/target_default_compile_warnings.cmake)
 include(cmake/target_default_compile_options.cmake)
@@ -75,6 +76,10 @@ if(FLECS_TESTS)
  add_subdirectory(test)
 endif()
 
+if(FLECS_M0DULE)
+ add_subdirectory(module)
+endif()
+
 message(STATUS "Targets: ${FLECS_TARGETS}")
 
 # define the install steps