Remove JupyterLab 2 support

[manics]

#108 converts the Python package to include a bundled JupyterLab 3 extension that does not require a build step when it's installed. In order to get this to work there I had to add some workarounds to package.json and to the GitHub workflow. This should be reverted when support for JupyterLab 2 is removed.

It should also be possible to upgrade typescript to 4.*, currently it fails with JupyterLab 2.