This is a no dependency, and possibly the world's smallest, wysiwyg editor.
https://github.com/jaredreich/pell/blob/master/README.md
If Flarum ever adds an official core editor extension, it would be cool if it was super tiny like this.
Edit: Oh, this was already mentioned here:
https://discuss.flarum.org/d/7145-i-found-a-good-wysiwyg-text-editor/11