Install jupyterlab before npm build

[davidbrochart]

check_release failed because jlpm could not be found, and since yjs-widgets is a JupyterLab extension it seems reasonable to add it as a dependency.

[jtpio]

A better way would be to install jupyterlab via a releaser hook instead, similar to the extension template:

https://github.com/jupyterlab/extension-template/blob/5696924f5b7adf07cf07ea9d3510c66914fbb770/template/pyproject.toml.jinja#L81-L87

That way yjs-widgets may have a chance to work in JupyterLite if we can avoid adding the dependency on jupyterlab.

[davidbrochart]

Thanks, I suspected there was a better way.