Add GitHub workflow to automatically update to newer JupyterLab versions

To ease maintenance, it would be useful to have a script that checks what the latest version of JupyterLab is, and automatically opens a PR that bumps to the newer packages.

Some ideas and possible config options for that script:

- run as a cron, and / or via a workflow dispatch on demand
- support updating to a specific version, `latest` (stable) or `next` (pre-release)
- this might also require being able to specify a branch, as multiple versions of notebook (stable and pre-release) can be developed at the same time

JupyterLab Desktop has something similar: https://github.com/jupyterlab/jupyterlab-desktop/pull/311

Start by reviewing the repository's current JupyterLab package declarations and GitHub workflow configuration. Define how stable, pre-release, and specific-version updates should be selected, how target branches are chosen, and what the generated pull request must contain. Done means the workflow can be run on schedule or on demand and opens the intended dependency update PR.

Written by the indexing model from the issue text.