P2918R2 Runtime Format Strings II

[StephanTLavavej]

WG21-P2918R2 Runtime Format Strings II
WG21-P3953R3 Rename runtime_format() To dynamic_format()

INCREASED feature-test macro, note that __cpp_lib_format has repeatedly increased through 202304L (#3799 WG21-P2510R3), 202305L (#3813 WG21-P2757R3), 202306L (#3808 WG21-P2637R3), 202603L (#4181 WG21-P2918R2):

#define __cpp_lib_format 202603L

Note: We are not yet accepting PRs for C++26 features.

[seishun]

I'm taking a stab at it in https://github.com/seishun/STL/tree/format.