Make Voici a federated extension

[martinRenou]

Problem

In #12:

The current approach this PR takes is to create a separate build directory in the JupyterLite output for Voici itself. This setup results in having a duplication of some core libs on the server. Instead, we should probably have the Voici app be a federated extension that we can load dynamically, and the voici main index should be similar to the retro one under JupyterLite (non-processed by webpack) . These improvements should be made in a follow-up PR.