Add bindings to `boon-emacs.el`

I've used these in my personal config for a while and think they are useful upstream as well. Each commit stands on its own, so feel free to mix & match as you see fit.

Regarding boon-switch-mark: x is shadowed by boon-command-map. @ was the shortcut for boon-switch-mark in version 1 of boon-emacs.el, so it feels like a natural choice to return to that.

Default keybindings in Emacs and the new bindings in this PR:

delete-indentation: M-^ => ^ in boon-command-map
indent-region: C-M-\ => \ in boon-command-map
scroll-up-command: C-v => v in boon-command-map
scroll-down-command: M-v => V in boon-command-map

jyp / boon

Add bindings to `boon-emacs.el` #133