Some jupyterlab extensions

[alimanfoo]

These look useful:

• https://github.com/jupyterlab/jupyterlab-toc
• https://github.com/mlshapiro/jupyterlab-flake8
• https://github.com/lckr/jupyterlab-variableInspector

This also looks great but maybe more involved to install:

• https://github.com/krassowski/jupyterlab-lsp