Added Keybindings to notebook editor

[jonah-iden]

What it does

This adds multiple keybindings and refactors some stuff to make it easier to add new ones.
This builds ontop of #13448 so that needs to be merged first

Adds the following keybindings:

• arrow up/down: change cell selection
• enter: edit selected markdown cell
• alt + enter: stop editing markdown cell
• ctrl+enter: execute selected code cell
• shift + enter: execute selected code cell and select the next
• Alt+O: clear outputs
• M: change selected code cell to markdown
• Y: change selected markdown cell to code cell
• A: Insert code cell above selected
• B: Insert code cell below selected
• d d: delete selected cell

How to test

Open or create a new notebook and test the keybindings

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

[jbicker]

Works and looks great! Thanks.