[whatwg/streams] ReadableStreamAddReadRequest() should assert state is readable (#454)

Currently `ReadableStreamAddReadRequest()` appends to the readers's `readRequests` list.  The `ReadableStreamClose()` function iterates this same list resolving its promises.  It would be nice to assert that we don't re-enter code that appends more items to this same list we are iterating.

I think the easiest way to do this is to add a state assertion in `ReadableStreamAddReadRequest()`.

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