Add new InputModel class for managing input state

[brichet]

This PR adds an InputModel class to handle input state.
This is proposed for:

• simplifying the use of the chats commands, that needs to get the input content, the current word and to be able to replace it.
• enhancing the messages editiing, by allowing adding and deleting attachments.

By default, the chat model includes an input model, for the "main" input of the chat panel. When editing a message, a new input model is created for this edition, and deleted when edition is over (cancelled or validated).

Fixes #163

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/input-model

[dlqqq]

@brichet I think this PR will cause some conflicts with the chat commands PR in #161. I think it would be best to rebase this branch & fix conflicts after merging #161. I'll give this a review after.

[dlqqq]

#148 should probably also be merged first to address any merge conflicts. I've approved that PR.

[dlqqq]

@brichet Wow, thanks for working on this so quickly! All of the high-level changes look good. I left some feedback for you below, but still need more time to complete a full review.

My day tomorrow will be full of meetings, so I will likely need until the end of Thursday to finish reviewing this. Feel free to work on another issue using the Kanban board: https://github.com/orgs/jupyterlab/projects/9

[dlqqq]

@brichet This looks perfect, thank you for building this!

[brichet]

Thanks for the review @dlqqq