Project reoganization

[brichet]

We should reorganize the repository with a python/extension directory and a packages directory (containing only the typescript sources).

This would probably clarify the repository, and would be more like other projects in the Jupyter ecosystem (https://github.com/jupyterlab/jupyter-collaboration, https://github.com/jupytercad/JupyterCAD).

In the meantime, we could also split the jupyterlab-collaborative-chat package, to expose the variables, classes and interfaces (at least the tokens). The plugins would reside in the extension side. That way other extensions could make use of it, and interact with the collaborative chat on the front-end side.