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

Implements the range-default-formatter specialization range_format::set.

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

Depends on D140801

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D141195