Rename to `jupyterlab-diff`

[jtpio]

With #16, this extension won't be about diffing cells only anymore, since it will also be able to diff files.

So we should look into updating the name to make it more aligned with what it does.

Here is the list of things that will need to be updated:

• Repo name: https://github.com/jupyter-ai-contrib/jupyterlab-diff
• References to the repo in the code base: Rename to jupyterlab-diff #20
• PyPI: create a pending trusted publisher after renaming the repo: https://pypi.org/project/jupyterlab-diff/
• npm: should be published automatically: https://www.npmjs.com/package/jupyterlab-diff