support variable inspector

[williamstein]

1. The upstream code is here: https://github.com/lckr/jupyterlab-variableInspector
2. For CoCalc we would make a new frame type. We can probably run a chunk of the upstream code, but obviously need our own view layer.

Workarounds:

• Use %who and %whos, which are built into Jupyter:

• You could use JupyterLab from CoCalc via +New --> JupyterLab Server, and then use the variable inspector JupyterLab extension if it were installed. It isn't right now, but maybe it will be soon.

[williamstein]

Also, due to realtime collaboration you can split the frame or open another popup by shift-clicking on the tab and update the variable inspector cell periodically by evaluating it:

[haraldschilly]

for jupyterlab, it is installed in the default 20.04 software environment. page name is lckr-jupyterlab-variableinspector.

[williamstein]

Closing this in favor of #5400