Migrate this extension from Jupyterlab 3.x to 4.x

[steff456]

Given that Jupyterlab 4 is out, now we need to migrate this extension for it to be compatible with the newer version. The oficial migration guide is this one, https://jupyterlab.readthedocs.io/en/latest/extension/extension_migration.html

I think that this migration is not going to be fast and users will remain using Jupyterlab 3.x for a while so we might need to support both versions for a while.