Use standard Typescript AsyncIterator types

[richardm-stripe]

This is a follow-up to #1739 (comment).

I was originally going to make the iterator return {done: true} instead of {done: true, value: undefined} (which would have been a breaking change), but it turns out that value: undefined is actually the standard:

> [][Symbol.iterator]().next()
{ value: undefined, done: true }

And it turns out that we can use the standard Typescript types for iterators/async iterators instead of defining our own. https://github.com/microsoft/TypeScript/blob/main/src/lib/es2015.iterable.d.ts#L11-L28
Doing this is nonbreaking.