Add Justfile support

[Dietr1ch]

There's justl that's quite handy on projects that use Just.

I recently started using these bindings and they seem to fit quite nicely

(spacemacs/set-leader-keys
  "pj" 'justl-exec-default-recipe
  "pJ" 'justl-exec-recipe-in-dir
 )

So far I added a private layer to further try it out, but it's looking like they'd be neat stock bindings.