Packaging JupyterLab extensions

What is the canonical way to provide JupyterLab extensions:
* For some extensions like jupyterlab-git, the package is a combination of a server extension written in python + frontend code in nodejs. Providing this as conda package makes the installation easier.
* other extensions like jupyterlab-plotly-extension are frontend only (contain no python part) and could instead also be installed via the JupyterLab Extension manager only...

In both cases JupyterLab requires to do a nodejs extension rebuild, which would be nice to have it already in a pre-build package, but afaik currently by design can not be done during the conda package build.

The npm stats show that many frontend extensions are installed from npm directly.

On the other side having frontend only extensions all with a version specified as part of an environment.yaml, is still appealing, even though the nodejs rebuild step+time is remaining. That is what came up during packaging and proceeding in https://github.com/conda-forge/staged-recipes/pull/8060

Already available packages:
* Server + Frontend https://github.com/conda-forge/jupyterlab-git-feedstock
* Frontend only https://github.com/conda-forge/jupyterlab-plotly-extension-feedstock

Potentially packages are e.g:
* Frontend only https://github.com/jupyterlab/jupyter-renderers (some are already integrated into default JuputerLab)
* Frontend only https://github.com/bokeh/jupyterlab_bokeh
* Server + Frontend https://github.com/dask/dask-labextension

To connect the dots, also linking to upstream https://github.com/jupyterlab/jupyterlab/issues/2065

Tagging @blink1073 @conda-forge/jupyterlab