Update highlight plugin interface
Update highlight plugin interface

I don't think this was on purpose. The `applyAction` interface is: `applyAction(action: Action, value: T, oldState: EditorState, newState: EditorState) → T`. By omitting the old value, the highlights were always operating on the old state, not the current one. This caused me some headscratches.

ProseMirror/website,ProseMirror/website