architecture: fix contentcomponent mounting

[YousefED]

Our EditorContent is a replacement for Tiptaps editorcontent. Although it cleans the original implementation quite a bit, there still seem to be some problems in how it's setup. See:

closes #750
closes #732

(Note that a workaround has been implemented here (#788), but I think that should actually be reverted when working on this issue (contentComponent should not be undefined whenever we call renderToDOMSpec).

My first thoughts are that we should clean the way the contentcomponent is set on the tiptap editor.

Ideas of better approach (open to suggestions):
a) useImperativeHandle instead of useEffect in EditorContent
b) when this EditorContent ref is available (in BlockNoteView) set it on the editor (as contentComponent) c) only call editor._tiptapEditor.mount` after step (b)

Of course, also need to take cleanup into account