support C++23 character range formatting

[dvirtz]

In C++23 it's possible to use the s formatter to format char ranges as strings

Format String	Contents	Formatted Output
{:s}	vector{'H', '\t', 'l', 'l', 'o'}	H llo
{:?s}	vector{'H', '\t', 'l', 'l', 'o'}	"H\tllo"

p2286r8

Trying to do the same with libfmt results in an "invalid format specifier" error

https://flux.godbolt.org/z/nacKGTfM7

[vitaut]

The s format specifier for ranges is not supported yet.