P2900R14 `<contracts>`

[StephanTLavavej]

WG21-P2900R14 <contracts>

This is a Core paper with Library impact.

We need to teach the IDE about this new extensionless header.

The Core feature-test macro is __cpp_contracts. Library feature-test macro:

#define __cpp_lib_contracts 202502L

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.