Implement polymorphic labels in the type system

[Marwes]

As described in #4410, there are many builtins that could get more precise type signatures with a way to abstract over record labels in the type system. #4388 sketches out such an type system extension, dubbed "Polymorphic labels".

Since updating the type signatures of the builtins may cause breakages we want to first implement and test the feature for new functions after which we can decide on a strategy for applying this to existing functions.

DOD:

• Decide and implement on a syntax for distinguishing a record field being a variable or a concrete field (Is A a variable or concrete "A" in { A: int })
• Provide alternate builtins in experimental for some of the functions that can use polymorphic labels for testing (both for running scripts and the LSP)
• A feature flag exists to turn on labels in the typechecker