Add list-timers to systemd user commands (#5099)

This is useful for listing "cron like" systemd jobs

plugins/systemd/systemd.plugin.zsh

@@ -1,6 +1,6 @@
 user_commands=(
   list-units is-active status show help list-unit-files
-  is-enabled list-jobs show-environment cat)
+  is-enabled list-jobs show-environment cat list-timers)
 
 sudo_commands=(
   start stop reload restart try-restart isolate kill