C++ 20 modules support

[MyJules]

We are using C++ 20, so introduce modules support so we can import vix;.

[GaspardKirira]

Thanks for the suggestion.

We agree that C++20 modules are the future of modern C++.

Modules support is planned, but will be introduced carefully to maintain
cross-platform stability across GCC, Clang, and MSVC.

An experimental import vix; umbrella module will likely be introduced
behind a CMake flag in a future release.

Backward compatibility with headers will remain fully supported.