[libc++][format] Finish P0645 Text Formatting.

This adjust the version macro and sets it as completed. All parts of the paper
have been implemented, except for the parts replaced by later papers and
LWG-issues.

Adjusted the synopsis to match the synopsis in the Standard. Not yet
implemented parts of P2216 and P2418 still use the P0645 wording.

Completes:
- P0645 Text Formatting

Depends on D115991

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D115999