Refactor undo/redo and integrate with playground

[amyworrall]

Lexical has an implementation of undo/redo, in History.swift. (Arguably this should have been in a Plugin, but there's some historical cruft here.)

Currently, to integrate history into a surface, you need to:

• register handlers for the commands undo, redo, and clearEditor, and in the handlers, proxy those commands to lexicalView.editorHistory
• create toolbar buttons to dispatch the commands undo and redo
• register handlers for canUndo and canRedo, and use these handlers to trigger updating the toolbar to enable/disable the undo/redo buttons
• register an update listener, that calls applyChange() on the EditorHistory instance, passing the active and previous editor states

I would like to see this improved. We should have a new EditorHistoryPlugin class, in its own package, which will:

• own and expose the EditorHistory object
• creates and keeps a reference to externalHistoryState, which is passed in to EditorHistory on init
• register the handlers for undo, redo, and clearEditor
  • In fact, you could move the declarations of the constants for the .undo and .redo commands into EditorHistoryPlugin as well
• registers the update listener that calls applyChange() on the EditorHistory object
• expose getters for canUndo and canRedo, which just check if externalHistoryState.undoStack > 0 (and same for redoStack). Note that this doesn't obviate the need for the canUndo and canRedo commands -- those are still useful to know when these values change.
• move History.swift into this package

Then, to integrate it into the playground:

• It should be enough to make the toolbar buttons just dispatch the undo and redo commands, now that the EditorHistoryPlugin is listening for them.
• Still register canUndo and canRedo, and use them to trigger a toolbar update.

Note, I've tagged this task good first issue, although there are quite a lot of moving parts. If you need any more context or help, let me know!