Fix notebook context selection

[msujew]

What it does

Closes #14178

Adds a tabIndex to one of the rendered html nodes of the notebook editor. This ensures that the context key service can find the correct active element when clicking on the toolbar button. If no focusable element is available, the browser will instead select the body element, which is linked to a different context.

How to test

Follow the steps from #14178 and ensure that things work as expected.

Please also ensure/test that all context key related stuff still works - i.e. j/k/up/down and y/m still work in command mode.

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

[jonah-iden]

Works 👍