Improve MSVC C++ modules compatibility for nlohmann_json (#4952)

* Improve MSVC C++ modules compatibility for nlohmann_json

- Keep complete module interface with all exports
- Export all standard types: json, basic_json, adl_serializer, json_pointer, ordered_json, ordered_map
- Export detail namespace symbols for advanced usage
- Ensure compatibility with MSVC C++20/23 modules
- Maintain full functionality without breaking changes

Signed-off-by: yudaichen <250074249@qq.com>

* Add C++20 module Windows CI test and fix file ending

- Add ci_module_cpp20 job to Windows workflow to verify module support with MSVC

- Add missing newline at end of json.cppm file

Signed-off-by: yudaichen <250074249@qq.com>

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Signed-off-by: yudaichen <250074249@qq.com>