Make a proper extension

[jtpio]

For now we override the CSS to hide the RetroLab extra UI elements with:

https://github.com/jtpio/nano-console/blob/805094013d07a749d3d540c9a6e9d1399ca06b80/index.css#L1-L13

This is not ideal. Instead we could make a small prebuilt extension that would customize the interface, so it gets properly deployed with jupyter lite build.

This would also be useful if we want to make a "Powered by JupyterLite" banner (similar to GitHub gists) at the bottom of the frame.