[feature request] Parse C++20 modules

[alfaix]

Hi!

I'm experimenting with C++20 modules in neovim and tree-sitter can't parse modules.
I took a crack at implementing it and it seems to work. Implementation can be found in this PR.

Creating this issue to avoid duplicating work.

[cdacamar]

I would love to see this. Currently translation units containing C++ modules keywords are hard to follow as highlighting quality quickly takes a nose-dive particularly after export is attached to various declarations.