[libc++][format] range-default-formatter for strings.

Implements the range-default-formatter specialization range_format::string
and range_format::debug_string.

Implements parts of
- P2286R8 Formatting Ranges
- P2585R0 Improving default container formatting

Depends on D145847

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D145853