Allow sxhkd-like shortenings

It could make our config files more concise to have those shortenings which sxhkd provides (see e.g. https://github.com/baskerville/bspwm/blob/master/examples/sxhkdrc).

So

```
cmd + alt - {1-9}: yabai -m space --focus {1-9}
cmd + {h,j,k,l}: yabai -m window --focus {west,south,north,east}
```

would be the same as

```
cmd + alt - 1: yabai -m space --focus 1
cmd + alt - 2: yabai -m space --focus 2
cmd + alt - 3: yabai -m space --focus 3
[...]
cmd + h: yabai -m window --focus west
cmd + j: yabai -m window --focus south
[...]
```

(previously accidentally posted in https://github.com/koekeishiya/dotfiles/issues/13)

Read the linked sxhkdrc example first and compare its shortening syntax with skhd's current config files. Define how brace expansions and paired value lists should behave for the shown space and window bindings, then verify that the requested concise forms expand to the equivalent explicit bindings.

Written by the indexing model from the issue text.