[libc++][format] Adds formatter floating-point.

This properly implements the formatter for floating-point types.

Completes:
- P1652R1 Printf corner cases in std::format
- LWG 3250 std::format: # (alternate form) for NaN and inf
- LWG 3243 std::format and negative zeroes

Implements parts of:
- P0645 Text Formatting

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D114001