P2637R3 Member `visit`

[StephanTLavavej]

WG21-P2637R3 Member visit

INCREASED feature-test macros, note special interactions with WG21-P2510R3 #3799 and WG21-P2757R3 #3813:

#define __cpp_lib_format  202306L // <== NOTE SPECIAL INTERACTIONS
#define __cpp_lib_variant 202306L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.