Abstact `ChatModel` in `@jupyter/chat`

[brichet]

I got a bit lost by the inheritance structure while reviewing this. We have IChatModel, then we have ChatModel implementing that interface (defined in @jupyter/chat), and then we have LabChatModel extending ChatModel in jupyterlab-chat.

As a reviewer, I was confused about why the Jupyter Chat project provides two implementations of IChatModel.

It seems like ChatModel in @jupyter/chat should be an abstract base class, and ChatModel should be renamed to AbstractChatModel. This would help make the inheritance structure more obvious. What are your thoughts on this?

Originally posted by @dlqqq in #148 (comment)