add C++20 module

[Arthapz]

No description provided.

[brandl-muc]

Given that constexpr has evolved substantially it might be a good idea to name the macro PUGIXML_CONSTEXPR11.
That would allow e.g. to have functions as constexpr in C++17 mode that could not be constexpr with C++11.

[Arthapz]

Given that constexpr has evolved substantially it might be a good idea to name the macro PUGIXML_CONSTEXPR11. That would allow e.g. to have functions as constexpr in C++17 mode that could not be constexpr with C++11.

good catch

[Arthapz]

here's a demo for C++20 module

https://github.com/Arthapz/pugixml_module_test