Fix interface definition of Generator object. It should extend IterableIterator<T>

[darcyparker]

The current definition is incomplete. It is both an Iterator and Iterable.

• See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#Is_a_generator_object_an_iterator_or_an_iterable

Also see #2873 (comment):

• "This has been revised: IterableIterator must be assignable to the type annotation instead."

• It references another pull request Basic support for generators as iterators #3031 which notes it should be IterableIterator<T>, but it neglected to update the type definitions for lib.es2015.generator.d.ts and es2015.generator.d.ts.

[alexisfontaine]

What about requiring all fields of the Iterator object?

Suggested change

	interface Generator<T> extends IterableIterator<T> { }
	interface Generator<T> extends Required<IterableIterator<T>> { }

The return and throw methods should not be optional as defined in the ECMAScript 2015 Language Specification. This would close #21527.

[rbuckton]

I am closing this PR as we are completely rewriting the Generator interface in #30790, which supercedes this change.