implement observable js notebook editor

[RandomFractals]

with new native vscode Notebooks UX and notebook document API for editing observable notebooks

[darknoon]

I started hacking on this in my fluid notebook repo.

Output-renderer branch has some work on getting HTML output to work (via returning the string instead of html…``, which needs document in the execution environment (looking as jsdom, etc).

[RandomFractals]

@darknoon thanks for sharing. I'll take a look.