[clang-tidy] Enable C23 support in modernize-use-nullptr (#89990)

C23 introduces the `nullptr` constant similar to C++11 which means that
the checker `modernize-use-nullptr` can be used on C23 code as well.

This PR enables the checker to be run on C23 and adds testcases.

See N3042:
https://open-std.org/JTC1/SC22/WG14/www/docs/n3042.htm