[red-knot] special-case returning NotImplemented

[carljm]

NotImplemented should always be returnable, no matter what return type annotation a function has.

Other type checkers seem to handle this by just treating NotImplemented as if it were Any, but I don't think that's right.

We can easily handle this by treating NotImplemented as a KnownInstance and special-casing it in return type checking.

(Arguably this special case should only apply to certain methods, but that seems more complex and perhaps not worth it.)