Add C++20 module

[Arthapz]

This PR add a C++20 module, enable examples with the module when EXAMPLES_USE_MODULE define is set
and add xmake build system support (mainly to test module compilation, it's easier to write and use than CMake module support and i'm not a CMake user / expert)

the FROZEN_DONT_INCLUDE_STL can be set to remove #include of std lib for user that want to use C++23 std module
we need this because mixing #include <stdheader> / import <stdheader> with C++23 std module doesn't work well atm on clang/GCC/msvc

to try it

install xmake: https://xmake.io/#/guide/installation

> xmake f --enable_tests=y --enable_module=y
> xmake b
> xmake run frozen.tests
> xmake run frozen.example.<example_name>

coverage is supported by using xmake coverage mode

> xmake f -m coverage