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.

[dlqqq]

In chat.tsx, we want to pass these 4 props to a new React context provider to allow children to access them via context hooks:

    /**
     * The rendermime registry.
     */
    rmRegistry: IRenderMimeRegistry;
    /**
     * The document manager.
     */
    documentManager?: IDocumentManager;
    /**
     * Chat command registry.
     */
    chatCommandRegistry?: IChatCommandRegistry;
    /**
     * Attachment opener registry.
     */
    attachmentOpenerRegistry?: IAttachmentOpenerRegistry;

There are also 2 extra unused props:

    /**
     * Autocompletion registry.
     */
    autocompletionRegistry?: IAutocompletionRegistry;
    /**
     * Autocompletion name.
     */
    autocompletionName?: string;

These unused props are part of the old autocompletion framework (implemented in #57) and can be removed. These are part of another issue: #168.

[brichet]

There are also 2 extra unused props:

/**
 * Autocompletion registry.
 */
autocompletionRegistry?: IAutocompletionRegistry;
/**
 * Autocompletion name.
 */
autocompletionName?: string;

Removed in #187