Support more JupyterLab distributions

[jtpio]

It would be interesting to see if ipylab can be used in a custom JupyterLab build (or just a frontend that reuses lab components), to create standalone a UI without all the other extra lab extensions (menu, left and right area...).

This would probably require making the ILabShell dependency optional:

ipylab/src/plugin.ts

Line 23 in 804d875

requires: [IJupyterWidgetRegistry, ILabShell],

[jtpio]

An example of such custom lab build could be JupyterLab Classic: https://github.com/jtpio/jupyterlab-classic

This would probably require making the ILabShell dependency optional:

But that would indeed mean some features exposed in Python like shell.expandLeft() would / should not be available in Classic.