Add TextEditChange event

[Zeophlite]

Objective

• There's no event when the pending TextEdit's are consumed

Solution

• Add the event

Testing

• CI

[viridia]

I'd like to see us gradually move in the direction of only triggering this event when there's an actual change, but we don't need to do that right now. For some kinds of text actions (cursor motion, copy), it's easy to tell that nothing has changed, whereas for other kinds (replacing selection with identical text, clearing an already-cleared buffer) are likely to remain false positives. The only way to do this perfectly is to keep around a copy of the buffer and compare them, which would be expensive. But I think it's reasonable to go after the low-hanging fruit.