Add support for uncontrolled ProseMirror component

[smoores-dev]

Adds a defaultState prop to support uncontrolled usage of the ProseMirror component! This PR ensures that useEditorState always returns an up-to-date EditorState, whether that's from the state prop or from the view state (if uncontrolled)

[tilgovi]

Do we need a separate prop for this or is there a way we can make it sufficient just to not pass dispatchTransaction, similar to how one can just not pass onChange handlers to regular form controls.

[smoores-dev]

I think these are separate, right? For native elements at least, passing onChange doesn't determine controlledness; whether value is undefined does (at least, I'm pretty sure). You might still want/need to hook in to dispatchTransaction to track some kind of state, even if you don't want to lift the whole EditorState

[tilgovi]

Your understanding is correct. I'm mistaken. React uses defaultValue and I had no memory of that, if I ever knew it. I think defaultState makes sense.