WG21-P2013R5 Freestanding Language: Optional ::operator new

This paper deliberately doesn't provide a feature-test macro, allowing WG21-P2198R7 #3794 to handle it.

I don't know whether there's anything for our hosted implementation to do here.

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.