Use correct cell type for selected language

[msujew]

What it does

Finishes the work of #13933 by adding a language selection to markdown notebook cells. When selecting a non-markdown language, any markdown cell automatically becomes a code block of that language.

How to test

1. Create a markdown cell in a notebook document
2. Assert that the cell displays a language selection during editing mode.
3. Change the language to a non-markdown one.
4. Assert that the cell type changes to a code cell (i.e. doesn't display markdown rendering, and is capable of being executed).
5. Changing it back is possible by selecting the markdown language again.
6. The change can also be undone by doing an undo action (via keybinding).

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

[jbicker]

LGTM. Works as expected.