[feat] Add optional keybindings

[sabbaticaldev]

Hey @antonmedv, thanks for the great work on CodeJar! It's been nice to work with it in comparison to the bloated monaco and codemirror. Nice to see an alternative.

I'm implementing it in a project of mine and would like to continue using some emacs keybindings on it so I started some silly POC implementation of it that can be found here: https://github.com/sabbaticaldev/codejar-emacs

and for that I implemented a simple keybindings logic on codejar. Feel free to reject or change it.

Thanks!