change cell type when selecting markdown as a code cells language

[jonah-iden]

What it does

Changes the cell type to a markdown cell when markdown is selected via the language picker in the bottom right of a code cell.

How to test

1. Open or create a new notebook.
2. Add a code cell and optionally write something inside
3. change its language to markdown via the language picker in the bottom right
4. it should correctly change to a markdown retaining the previous content and have the ability to render the markdown

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me. Currently we don't have a way to go the reverse direction (i.e. from markdown -> code), but that's out of scope for this PR.

[jonah-iden]

yeah at some point we probably need to add language selection for makrdown cell as well. There is a command/keybinding to change markdown cell to code cell and back though