Restore the helper text in input

[brichet]

This PR restore the helper text containing the shortcut to send a message.

Part of #303

[github-actions]

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

[jtpio]

Thanks for looking into adding this back.

Wondering what this would look like when the toolbar has many buttons, like in jupyterlite/ai#218?

Otherwise, maybe we don't really need to add it back?

[brichet]

Otherwise, maybe we don't really need to add it back?

I often find it useful, each chat system has it's own way to add a new line, and I'm often confused. In the context of AI, it's annoying to send an incomplete message when you only wanted to add a new line.
But maybe we can show this information in a different way, like a tooltip on the send button for example.