Problem

A new chat only display the input in the bottom, without any instruction.
See jupyterlite/ai#44 for more details.

We should be able to add a custom default message when opening a new chat, for e.g. instructions related to AI configuration.

Proposed Solution

Add a welcome message on top of all the messages of the chat. This welcome message should be customizable from the extension that use the chat, using the markdown renderer as text renderer, allowing links and images.

Additional context

Initial issue jupyterlite/ai#44