Install core packages from melpa-stable

Use-package broke in a recent update, as with Doom, since it relies so
heavily on it. To combat this issue arising again, use-package will be
installed from melpa-stable from now on.

Addresses #283

core/packages.el

@@ -4,6 +4,7 @@
 ;; core packages
 (package! s)
 (package! f)
+(package! async)
 
 ;; core-os.el
 ;; In case this config is shared across multiple computers (like mine is), let's