Test for mimerender and theme extensions breaking changes

[jtpio]

Problem

There has already been some discussions about testing for breaking changes (will add links to this issue later as needed if any).

However most of the time this requires manually checking and thinking about the implications of the proposed changes in a PR.

Ideally there could be some automated check that would help catch such changes.

Proposed Solution

Maybe the existing Galata tests could be extended to also check for third-party mimerender and theme extensions.

The main issue would be to choose such third-party extensions that are considered stable enough and maintained to be included in the automated test suite. For example using one of the extensions from https://github.com/jupyterlab/jupyter-renderers. For the theme maybe there could be a "semi-official" one under the jupyterlab organization (or generated from https://github.com/jupyterlab/extension-cookiecutter-ts).

The alternative is to remember to test with such third-party extensions during the JupyterLab alpha cycle to catch potential issues early.

Additional context

Linking to #13461 as an example of an unintentional breaking change for third-party theme extensions.

Also linking to #12581 which unbumped the major version of rendermime-interfaces to avoid extension authors to manually update their extension for JupyterLab 4.0.

[jtpio]

Adding to the 4.0 milestone for consideration (although not blocking)