Improve the loading of extensions in `index.js`

[jtpio]

Problem

For now the packages are required in the main index.js file:

https://github.com/jupyterlab/retrolab/blob/4221275e895bb5f6be57c6ed85d7b1058eb9dde0/app/index.js#L83-L143

This is a bit tedious as this file needs to be updated when we add new dependencies to the package.json.

Proposed Solution

Maybe we don't want to have a different set of extensions loaded on the page depending on whether we are using the notebook, console, file browser or terminal? And instead have a single list defined in the package.json.

This would make it easier to streamline that part of the code, and hopefully make some of the boilerplate for loading federated extensions more reusable.

Additional context

This might also help with: jupyterlite/jupyterlite#118 (comment)