Treat spaces as tabs on backspace

One thing I miss from Sublime is its ability to treat spaces as if they were tabs when backspacing. I've had a go at implementing this functionality, as well as making it play nicely with the existing auto-parens behaviour.

I couldn't find a built in way to compose two behaviours into a single keybinding like this, so `keyboard/raise` is my attempt at providing one. Let me know if you want any changes.