Add ability to trigger undo/redo programmatically

[BertalanD]

Checklist

[x] Are you reporting a bug? Use github issues for bug reports and feature requests. For general questions, please use https://discuss.yjs.dev/
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Is your feature request related to a problem? Please describe.

I am looking for a way to trigger undo/redo programmatically (i.e. from a button click event handler).
It should be doable given only an EditorView instance, I don't want to pass around the UndoManager instance as well.

Describe the solution you'd like
I think the easiest fix would be to export y-undomanager.js's undo/redo functions from this library.