Subissue: Allow command providers to be initialized async

[dlqqq]

Problem

Chat command providers may need to make a network request to get its list of commands when the application starts. For example:

• Jupyter Chat's emoji provider could make an async dynamic import (e.g. via import(...)) to import a library package of emojis.
• Jupyter AI's slash command provider needs to fetch the list of slash commands from the backend.

Proposed Solution

Add a ready: Promise<void> property or an initialize(): Promise<void> method to IChatCommandProvider.

Additional context

None.