[libc++][format] Updates to Unicode 15.

This adds support for the new code points in the Extended Grapheme
Cluster algorithm. The algorithm itself has remained unchanged.

The width estimation still follows the rules of the Standard.
@cor3ntin filed
  LWG3780 format's width estimation is too approximate and not forward compatible
to improve the estimate.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D134106