CXX-20 MODULES - global module fragment support

[alexv-ds]

This PR is based on one sleepless night and a similar PR for entt.

The problem is that we get compilation errors when using the methods (when include flecs.h in global module fragment):

world.import<ModuleName>();
world.system<ComponentName>();
// And perhaps others that call some template functions, the corrections of which are given in this PR

The reason to be that certain function declarations that are implicitly required by the implementation aren't exported into the module interface since they are marked 'static' and have internal linkage.

And when compiling we get the following error (clang-18 ubuntu 24.04 package version 1:18.1.3-1):

# This is an incomplete output for readability purposes.
/home/alexv/Desktop/flecs/include/flecs/private/../addons/cpp/mixins/term/../../utils/signature.hpp:44:25: error: no matching function for call to 'type_to_inout'
   44 |             , inout ({ (type_to_inout<Components>())... })
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~

This pull request solves this problem. I also advise to look at the original pull request for entt (link at the beginning of the description)

[SanderMertens]

LGTM!