Use DiffEditor DiffNavigator to navigate diffs

[colin-grant-work]

What it does

Fixes #14888 by adding diff navigation commands to the toolbar of diff editors and using the existing diff navigator tool in MonacoDiffEditors to navigate among changes.

How to test

1. Open a diff editor with some changes.

Common ways to do this: modify a file and then open it from the source control view; use the 'select for comparison...compare with selected' commands in the file tree; using the AI chat features, ask the @Coder agent to make modifications.

2. You should see up and down arrows on the toolbar, and clicking them should take to the next / previous change in the file, as appropriate.
3. If you open a diff editor with no differences, the arrows should appear, but they should be greyed out.

@ reviewers, there was relevant work in #13104, but since that was mostly related to @theia/git, I'm not sure the machinery implemented there is relevant anymore. The code here just skips that machinery in the case that I believe I know how to handle better, but possibly it's just no longer necessary. In particular, I'm not sure how we would have a diff in an editor with a file scheme.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines