Improve notebook editor performance

[jonah-iden]

Feature Description:

Currently the notebook editor renders a full monaco editor for each code cell. This is quite the performance impact, especialy with large notebooks, containing lots of code cells.
Vscode is fixing this by only rendering cells/editors that are currently visible in the viewport.
Another possible fix could be to only render cheap fake editors and replace them with real ones when a user starts editing.
Maybe even both solutions could be done.