Allow spreading arrays after required parameters

[sandersn]

Fixes #5296 using the simple method described briefly in at the bottom #15747.

Specifically, this treats arrays that are spread as an infinite list of all-optional arguments. This lets you spread arrays into all-optional parameters whose types match.

This simple definition has a few more implications:

1. You can spread arrays into parameter lists that are too short (and whose body presumably uses arguments) [1].
2. Length is not checked, even if it is known.
3. Tuples are not treated specially.

Both of these might come later, but would probably require #6229, @Igorbek's strict-length tuple proposal. And much more code, since this is a one-line change.

[1]

declare function mungeArguments(): string;
declare const stuff: string[];
mungeArguments(...stuff) // ok!

[KiaraGrouwstra]

Minor correction: that's #6229 rather than 6629.