Guidance for `readyState` fields

[LeaVerou]

These are all over the place.

Types:

• FileReader, EventSource, HTMLMediaElement, IDBRequest, XMLHttpRequest, WebSocket use numbers (as static constants on the class)
• Document, MediaSource, RTCDataChannel use strings

There is the existing guidance to use strings for enums, though number constants do have the nice property that comparisons are possible (e.g. this.readyState >= Foo.LOADING), and seem much more prevalent.

The states themselves are also all over the place:

• Document: loading, interactive, complete
• MediaSource: closed, open, ended
• FileReader: empty, loading, done
• EventSource: connecting, open, closed
• HTMLMediaElement: have nothing, have metadata, have current data, have future data, have enough data
• HTMLTrackElement: none, loading, loaded, error
• IDBRequest: pending, done
• XMLHttpRequest: unsent, opened, headers received, loading, done
• WebSocket: connecting, open, closing, closed

Of course these cannot be completely harmonized since the use cases are different, but I suspect there is some guidance that can be extracted to apply to future classes…