Add back the `-` alias to go to the previous directory

The alias was removed in #3564. A couple of users were using
it, so I'm adding it back.

lib/directories.zsh

@@ -8,6 +8,7 @@ alias -g ....='../../..'
 alias -g .....='../../../..'
 alias -g ......='../../../../..'
 
+alias -- -='cd -'
 alias 1='cd -'
 alias 2='cd -2'
 alias 3='cd -3'