Add C++20 module build target v2

[Gaspard--]

This is a cleaner attempt at #1364 which hopefully is more maintainable and approchable.
While #1364 tried to tag each symbol, this PR tags namespaces instead, reducing the amount of defines that need to be inserted.

Usage in client cmake:

set(FLECS_M0DULE ON CACHE BOOL "Build flecs module" FORCE)
add_subdirectory(path/to/flecs)
target_compile_features(flecs-module PUBLIC cxx_std_23)
target_link_libraries(
   someProjectName
   PRIVATE
   flecs-module
)

In the code, it is then possible to use import flecs; to import all the flecs symbols. The current version includes everything in the flecs namespace, but avoids including anything in the _ namespace.

Maintainance:

• namespaces that contain symbols to exported must be tagged with FLECS_API_NAMESPACE
• namespaces that should be excluded should be seperate in order to avoid exporting symbols
• static globals must be tagged with FLECS_STATIC_IN_HEADER, which becomes static in headers, but not in the module
• some rare symbols outside namesapces are necessary for compilation, such as operator new / delete, and must be tagged FLECS_API_DEPENDENCY

I'd ideally like to find a way to modify the tests to import flecs instead of including it, but I don't know enough about bake to know if it's even possible (which seems unlikely if bake only works with includes).