Re: [whatwg/webidl] Add `async iterable<T>` type to WebIDL (PR #1397)

`AsyncIterator.from` will accept strings because `Iterator.from` does. But do note that it's specifically a coercion method. For something like `ReadableStream.from`, which is also a coercion method, you may wish to follow that precedent, but I'm not sure you'd want to do so for other async-iterable-taking things.

The way `AsyncIterator.from` works is to look up `Symbol.asyncIterator`, and if that fails then fall back to `Symbol.iterator` with the result wrapped the same way `for await` does (modulo some quibbles not relevant here), and if _that_ fails to use the `.next` method directly (which only _iterator_-taking things should do, which is to say, nothing except this specific method and `AsyncIterator.prototype.flatMap`).

And since it will not specifically reject strings, and strings have a `Symbol.iterator`, it will accept them, the same way `for await` does.

(Sorry the spec for `AsyncIterator.from` is out of date; I'm working through [other parts of the semantics](https://github.com/tc39/proposal-async-iterator-helpers/issues/4) and it doesn't really make sense to get the spec updated until those are sorted. But this question, at least, is settled.)


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