feat(fancy-ctrl-z): Flip-flop between two backgrounded jobs

Added a feature to `fancy-ctrl-z` which, if and only if there are
precisely two backgrounded jobs, switches between them (using `fg %-`)
when bringing back something to the foreground.

plugins/fancy-ctrl-z/fancy-ctrl-z.plugin.zsh

@@ -1,12 +1,18 @@
 fancy-ctrl-z () {
   if [[ $#BUFFER -eq 0 ]]; then
     BUFFER="fg"
+
+    IFS=$'\n' local num_jobs=($(jobs))
+    if [[ "${#num_jobs[@]}" -eq 2 ]]; then
+      BUFFER="fg %-"
+    fi
+
     zle accept-line -w
   else
     zle push-input -w
     zle clear-screen -w
   fi
 }
+
 zle -N fancy-ctrl-z
 bindkey '^Z' fancy-ctrl-z
-