Support Diff-based edits

[JoeHowarth]

When working with a large file, it's not practical to have the model re-write it entirely for each edit. Many models support Search + Replace blocks that would make edits faster and make using more powerful and expensive models practical.

For a reference example, look at aider's feature.

[bracesproul]

You can do this with the highlight to edit feature. On the backend, that takes the highlighted text, along with 200 (if I remember correctly, it's somewhere around there) characters before & after and only regenerates that.