✨ Vim-style modal editing

(First of all thanks for this awesome project, looks amazing already and I can't wait to play with it more.)

**Is your feature request related to a problem? Please describe.**
As a Vim user it would be great to have modal editing with Vim keybindings.

**Describe the solution you'd like**
A vim mode, similar to [VSCodeVim](https://github.com/VSCodeVim/Vim) or Xcode 13's new Vim mode.

**Additional context**
Perhaps this could be done via the proposed extension API, once ready:
- #76