run `onBeforeTextEvent` also for undo and redo events

[matthias314]

The onBeforeTextEvent is not documented, but it has been discussed here, for example. Currently this callback is only called for "new" events, but not for those coming from undo or redo events. This means that these latter events are missed if one uses onBeforeTextEvent to monitor how a buffer changes. This PR fixes this by moving the invocation of the callback to a function that is also called for undo and redo events.

[usfbih8u]

@Andriamanitra this could be relevant to Micro LSP if it is merged.