Support forthcoming changes to Iterator type in TS 5.6

[rbuckton]

In an effort to clean up the definitions of Iterator, IterableIterator, and Iterable (as well as their Async variants) to make way for types for the ECMA262 Iterator Helpers proposal, microsoft/TypeScript#58243 makes a few changes to these types that result in several breaks in some type packages.

This has been authored as a Draft PR for now, as microsoft/TypeScript#58243 is still in flux and has not yet been merged.

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