Add functionality to export chats

[srdas]

In Jupyter AI v2, the /export command enabled writing the contents of the chat to a markdown file with a timestamp. In v3 it would be nice to have similar functionality. I was using the export feature quite extensively in v2 to generate symbolic math solutions to differential equations, which run into a page or more and then directly take the output into my LaTeX documents. Since I need this feature frequently, I wrote a small CLI python program to do this (attached here), but hoping this will be replaced eventually by a new feature.

chat2md.py.TXT

Some possible ways in which the export feature can be implemented in v3:

1. Reintroduce the /export command. [Easy]
2. Enable free form language requests to export the chat in any of several formats (default to markdown). [Hard]
3. Add a export button to the chat tab (default to markdown, but can also reveal a drop down list of formats). For example, the cline library has an EXPORT button. [Medium]