Emacs keybindings in the val.town editor.

[saolsen]

I think this would be pretty easy to add and would make my life a whole lot better. :)

• Add a new advanced setting (similar to Vim Keybindings).
• If it's set, enable this codemirror extension https://www.npmjs.com/package/@replit/codemirror-emacs.

[tmcw]

✨ This is now shipped: we now support default, vs code, emacs, and vim keybindings. Big thanks to Replit, which is maintaining the open source CodeMirror extensions for each of these!