[libc++][format] Add basic_format_parse_context.

Implements parts of:
- P0645 Text Formatting

Depends on D92214

Reviewed By: ldionne, curdeius, #libc

Differential Revision: https://reviews.llvm.org/D93166

GitOrigin-RevId: 35a57f39b5d126646335183f1ff0f7adb52d63bc
8 files changed