Use a JupyterlabContext React context

[brichet]

Would it be helpful to build some kind of JupyterlabContext React context which provides access to all the APIs provided by other JupyterLab extensions?

For example, I was thinking that a component could call something like:

const { commandRegistry } = useJupyterlabContext();

and similarly so to access other JupyterLab singletons. This would simplify the frontend code by not requiring us to pass every singleton as a prop to every child component. This is also equivalent in terms of performance, as these singleton objects are not changed at runtime.

Originally posted by @dlqqq in #148 (comment)

This context could probably includes most of the properties of the ChatBodyProps, but also the activeCellManager and the selectionWatcher that are currently in the model.

[dlqqq]

Agreed! There may be other things we should provide through that context. rendermimeRegistry comes to mind.