Make streamed changes apply as a single History event

[andrewpareles]

We recently added in streaming so that the AI's edits are gradually displayed on the screen. The problem is, each edit counts as its own write event in VSCode's history, which means the user needs to press Ctrl+Z many times if they want to undo the AI's changes.

We'd like an AI's change to count as a single event in the history.

This may be related to #57.