Add a welcome message

[brichet]

This PR adds an optional welcome message to the chat, rendered as markdown.
This message is displayed on top of all the messages in the chat widget.

It also adds a token to jupyterlab-chat, to allow third party extensions to provide their own message with a plugin. There is no default message in jupyterlab-chat.

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Fixes #217
Related to jupyterlite/ai#44

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/welcome_message

[brichet]

I was hoping to add UI tests on welcome message, but I'm not sure if this is possible, since jupyterlab-chat doesn't provide a default welcome message.

The 2 ways I could think of to test it are:

• adding a plugin that provides the expected token IWelcomeMessage before opening jupyterlab (I don't think this is possible)
• testing the jupyterlite deployment built in the doc https://jupyter-chat--221.org.readthedocs.build/en/221/lite/lab/index.html

[brichet]

Bump CI

[jtpio]

Looks like this method is new. Would it make sense to pass an options object instead of individual arguments? In case more parameters need to be added later.

[brichet]

Right, I updated it.

[brichet]

While updating the renderContent function, I noticed that this new namespace MarkdownRenderer may be more relevant outside of the components directory, for consistency (this is not a component).
I moved it in it own module, while keeping the MessageRenderer in the components.