[libc++][format] Adds parser std-format-spec.

This implements the generic std.format.spec framework for all types.

The Unicode support will be added in a separate patch.

Implements parts of:
- P0645 Text Formatting

Completes:
- LWG-3242 std::format: missing rules for arg-id in width and precision
- P1892 Extended locale-specific presentation specifiers for std::format

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103368