Adding split-file diffs

[nakul-py]

This PR adds split-file view for diffs.

Screencast.From.2025-11-04.21-06-17.mp4

CC @jtpio @Zsailer

[nakul-py]

Hey! @jtpio and @Zsailer I need more suggestions from you about working of panes and need of accept-reject buttons in split-file view.
Currently both left and right panes are view only both editors are non-editable i am thinking
to make right pane editable and also the revert changes button per block or lines of code changes like we got in vs code split-file view.

[nakul-py]

74eb8c0 This commit make right pane editable and syncs edits in the right pane diff editor into the actual JupyterLab file document.

Screencast.From.2025-11-04.16-02-44.mp4

[nakul-py]

I have updated the PR description video.

[brichet]

Thanks @nakul-py for working on this, this is promising.

This is a very quick review, mostly about consistency, according to the screenshot below.

• The filepath can be confusing
  In this case, I provided a fake filepath, and the diff widget opened with the current editor filepath (which is wrong).
  Applying the changes applies them to the current editor, in this case markdown.md instead of anything.txt (as the command suggest).
  We should probably throw an error if the file path is not correct.
  I wonder if we should check that the file content is one of the originalSource or newSource for consistency.

• Undo / Redo buttons
  AFAIU we can currently only accept or reject all changes. If this is true, I don't think the Undo/Redo buttons are necessary because the widget close after the action.
  I don't know how difficult it is, but it would be wonderful to be able to accept and reject each diff independently. Otherwise these buttons can probably be removed for now.

[brichet]

I don't know how difficult it is, but it would be wonderful to be able to accept and reject each diff independently.

Just saw #37 that should fix it, thanks 🎉

[nakul-py]

The filepath can be confusing
In this case, I provided a fake filepath, and the diff widget opened with the current editor filepath (which is wrong).
Applying the changes applies them to the current editor, in this case markdown.md instead of anything.txt (as the command suggest).

Currently this case is also valid for unified-file-diff.