Custom editor will randomly not use the custom editor

[rchiodo]

Enable custom editor
Create a blank notebook

This comes up:

We might fix this by using openWith instead of open [Edit], we already use openWith so that's not the problem.

[rchiodo]

Blocked on:
microsoft/vscode#100238

[greazer]

Looks like Matt thinks this is something we can address.

[rchiodo]

In VS code insiders, the new notebook stuff is fixed.

Still left to do:

• Check undo for all operations. Seems to break on move cells

[rchiodo]

This is only working in VS code insiders at the moment.

[IanMatthewHuff]

Validated. Not sure how common it is. But I tried it about a half dozen times and didn't get the error. Great to not have the raw ipynb popping up any more as well.