fix(heroku-alias): add command guard check for heroku

- Add command existence check at the top of the plugin to prevent
  aliases and functions from being created when heroku is not installed.
  This is consistent with other plugins (e.g. bun, volta, gh, helm)
  that guard against missing commands.

plugins/heroku-alias/heroku-alias.plugin.zsh

@@ -1,3 +1,8 @@
+# Return immediately if heroku is not found
+if (( ! ${+commands[heroku]} )); then
+  return
+fi
+
 # general
 alias h='heroku'
 alias hauto='heroku autocomplete $(echo $SHELL)'