uchar.h: add char8_t

[fuz]

Summary

A type similar to char16 and char32_t, for compliance with C23.
The related type atomic_char8_t is added to stdatomic.h.
As char8_t is always unsigned char, I've skipped adding __char8_t.
This can be added, too, if desired.

[imp]

I'd be tempted to do separate commits, but it's not therrible if not. The change looks good to me.