[libc++][format] Adds char formatter.

Implements the formatter for all fundamental integer types.
[format.formatter.spec]/2.1
The specializations
```
  template<> struct formatter<char, char>;
  template<> struct formatter<char, wchar_t>;
  template<> struct formatter<wchar_t, wchar_t>;
```
This removes the stub implemented in D96664.

Implements parts of:
- P0645 Text Formatting

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D103466