Add Iterator methods

[Elchi3]

https://github.com/tc39/proposal-iterator-helpers
https://chromestatus.com/feature/5102502917177344
https://v8.dev/features/iterator-helpers

Named "Iterator methods" for consistency with https://github.com/web-platform-dx/web-features/blob/main/features/set-methods.yml but happy to name it "Iterator helpers" ("helpers" feels quite generic, though). Also, I listed all the additions in the description just like we did with "Set methods".

[autonome]

Pre-review.

Agree that "helpers" sounds generic, even though it's named that in the spec. Maybe worth digging around to see how developers refer to them, if you haven't already. But a convention around "methods" sounds good to me, as it's likely to come up again, and also naming is a balance of the status quo and what might make things better/easier for webdevs.

[ddbeck]

Just for posterity: I wondered by javascript.builtins.Iterator wasn't included here. Ultimately, I found that it makes sense: BCD tells a half-truth in that all browsers support Iterator in the sense that they implement iterators. But don't actually expose Iterator as a defined object, at least not until they ship Iterator(). The BCD and MDN docs are extremely confusing on this point.