[whatwg/url] Does URLSearchParams#keys() return an Iterator or IterableIterator? (#521)

I tried to answer this by looking at the spec, which just says `URLSearchParams` itself is iterable: https://url.spec.whatwg.org/#interface-urlsearchparams

I then looked at [the definition `iterable` in the IDL spec](https://heycam.github.io/webidl/#idl-iterable), but it's still not clear to me  whether the return value of `keys()` itself is iterable.

I also checked the [webplatform tests](https://github.com/web-platform-tests/wpt/tree/master/url) and couldn't find any test for this.

I noticed that Chrome and Safari return an iterable, meaning you can do this:
```ts
const params = new URLSearchParams('?foo=bar')
for (const param of params.keys()) {
  console.log(param)
}
```
The same code in Firefox (77.0b6) throws a `TypeError: searchParams.keys() is not iterable`, so this is a web compat issue, but I don't know who is behaving correctly here.

As a consumer I have a use case for iterating `keys()` with `for of` so I'd prefer it to be iterable, and `Map`, `Set`, `Array` etc all behave this way.

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