Add Option to Clear Notebook Outputs Before Commit

[Meriem-BenIsmail]

Fixes #392.
This PR introduces an optional workflow to help users keep cleaner Git histories when working with Jupyter notebooks.

1. Clear-Outputs Prompt Before Commit
   When committing, if staged notebooks contain cell outputs, a dialog now appears offering two choices:

Screencast.from.2025-11-27.09-39-48.webm

2. Git Setting: Always Clear Outputs
   A new configuration option has been added to the Git extension settings:
   When enabled, notebook outputs are automatically stripped before every commit, skipping the dialog entirely.

Implementation Details
Backend endpoints were added to:

• detect staged notebooks with outputs.
• clear outputs using jupyter nbconvert.

[github-actions]

👈 Launch a Binder on branch Meriem-BenIsmail/jupyterlab-git/clean-outputs

[afshin]

Thank you Meriem!