Add C++ modules support and update nlohmann::json

[mikomikotaishi]

This pull request adds support for C++20 modules.

In order to accomplish this, some constants were changed to using external linkage. I have also updated nlohmann/json to version 3.12.0, as the older version declares some symbols as internal.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

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