Only initialize notebook cell editor when in viewport

[jonah-iden]

What it does

This implements parts of #13015
This optimizes the notebook editor widget so that monaco editors for code cells are only created once they come into the viewport. This should make opening large notbooks quite a bit more performant.

How to test

Open a notebook. Everything should still look the same. You should be able to see in the dom tree that all except the editors in the view are only mock editors with the estimated height of the content

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]

The editors are empty after I've reloaded the page and have another editor focused first. When I scroll everything is shown.

notebook-editors-empty.mp4

[jbicker]

Besides that everything works very fine.