[Lint] Add a rule to replace .forEach with a for-in loop

[xedin]

If there is no chaining using for-in loop more understandable and requires less chaining/nesting.

[allevato]

Looks good, thanks!

I tend to be a bit wary about introducing syntactic rules that technically require semantic information to prevent false positives. To be 100% accurate we'd want to check that the forEach reference is a Sequence.forEach witness; it's easy to think of a type that's not a Sequence that introduces its own forEach method that this could diagnose on.

But that might be rare enough that we shouldn't worry about it, unless it becomes a problem in practice. And since this is only linting and not rewriting the tree, the worst-case outcome is that someone in that situation would have to add a swift-format-ignore comment.