Watch mode for non-federated third-party extensions

[jtpio]

Description

Follow-up to #9101 and #9089

Running jupyter lab --no-browser --watch doesn't seem to pick up changes when the extension is built.

Reproduce

Create a new environment and install lab with python -m pip install --pre jupyterlab.

Tested while updating the @jupyter-voila/jupyterlab-preview extension to support 3.0, but without switching to the new structure (yet): voila-dashboards/voila#732

The extension seems to be picked up and linked correctly though:

JupyterLab v3.0.0rc2
Other labextensions (built into JupyterLab)
   app dir: /path/to/miniconda/envs/voila/share/jupyter/lab
        @jupyter-voila/jupyterlab-preview v1.1.0 enabled OK*


   local extensions:
        @jupyter-voila/jupyterlab-preview: /path/to/git/voila/packages/jupyterlab-voila

Context

JupyterLab 3.0.0rc2

[jtpio]

With the build command being the one from the 2.0 cookiecutter (running tsc):

https://github.com/jupyterlab/extension-cookiecutter-ts/blob/cb9ca8c37bf1c5c28be61b31e6fcd0cf60487a5f/%7B%7Bcookiecutter.extension_name%7D%7D/package.json#L30

[telamonian]

The easy fix will be to just do a union regex. This will need some testing, but then I'll open a PR:

WEBPACK_EXPECT = re.compile(r'(?:.*/index.out.js)|(?:.*theme-light-extension/style/index.css)')

current:

jupyterlab/jupyterlab/commands.py

Line 40 in 14bcbea

WEBPACK_EXPECT = re.compile(r'.*theme-light-extension/style/index.css')