Add support for C++20 modules

[mikomikotaishi]

I made a similar pull request here on RtMidi, and am just applying the same edits here.

This pull request adds support for C++20 modules, as well as puts all symbols in the inlined namespace rt::audio (it is broken into nested namespace to emphasise the relation with RtMidi, which also shares namespace rt). This change does not break any existing code, but allows symbols to be optionally accessed through the namespace rt::audio in addition to the global namespace. The module itself is imported using import rt.audio; and is enabled with the build macro RTAUDIO_BUILD_MODULES. Enabling RTAUDIO_USE_NAMESPACE will make namespaces mandatory in the module.

Introducing modules to the project will not break any code, but is a benefit to users of C++20 and later who may wish to consume the library using modules rather than headers.

[mikomikotaishi]

Ah, so it seems including the inline namespaces in RtAudio.cpp was not only unnecessary but even directly preventing it from compiling.