Export conversation to markdown

[Steve235lab]

Describe the changes you have made:

Add a new magic command %markdown [path] to export the conversation to a specified Markdown path. If no path is provided, it will be saved to the Downloads folder with a generated conversation name.

Reference any relevant issues (e.g. "Fixes #000"):

Pre-Submission Checklist (optional but appreciated):

• [x] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [x] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [ ] Tested on Windows
• [x] Tested on MacOS
• [x] Tested on Linux

[Steve235lab]

Originally, I want to export the conversation to PDF, however it would import new dependencies, so just markdown for now. If you think exporting to PDF is also needed, it's also easy to extend in the future.