[rdf] add support for snippet byte_range and line_range

[meretp]

#273 adds the fields byte range and line range to the internal data model. The rdf parser/writer need to be updated to also handle these new fields. Once this functionality is added the initially skipped tests in test_conversion.py and test_write_anything.py should be reactivated. And we should also add validation functions for the new fields, at least for the mandatory byte range field.

[armintaenzertng]

Full SPDX 2.3 support is now part of the refactored version found on main.