When considering new standard library functions, we essentially need to strongly justify their existence over their implementation in Luau in user library code.
This PR attempts to provide a few axes of consideration; ideally new library functions tick many of the boxes, eg "used often + is more performant + clear unambiguous interface" is an ideal consideration for a library function, whereas if it's merely accelerating a single specific use case for a single application it's unlikely to be a good justification for inclusion.
It seems more consistent and unambiguous if we mark RFCs as being
implemented when the implementation lands instead of expecting to
cross-reference documentation. That also makes it easier for us to flag
stale RFCs.
This finishes the set of fully baked/accepted RFCs; 3 more proposals have been accepted but not implemented yet and will be submitted separately as actual RFCs.
