Adding Arrow button to revert a single block changes in cell

[nakul-py]

Adding Arrow button to revert a single block changes in cell like something we have in vs code split view

Screencast.From.2025-11-10.17-48-02.mp4

cc @jtpio @Zsailer @brichet

[brichet]

Thank @nakul-py for working on this, I like it !

I played with it and have some comments:

• we should probably close the compare widget if all the changes have been reverted. See the image below, there is no more change, but the comparison is still opened.

  

• there is an error when trying to revert the changes in the last line of the cell

  Uncaught RangeError: Invalid change range 40 to 57 (in doc of length 56)
  

• not for this PR, but we should probably display the removed line in the right panel, otherwise it is unclear that the line has been removed (see line 3 below)

  

[nakul-py]

Thanks! @brichet for the deep review. I will look into it soon.