allow user to extend or overwrite omarchy-menu

[DarrenVictoriano]

This PR introduces a supported extension point for Omarchy menus by sourcing
menu-extensions.sh after all core menu functions are defined.

By loading extensions last, users can override or extend menu behavior
(e.g. show_system_menu) without modifying the main script.

This enables customization while keeping upstream updates intact.

example:
user to create this file ~/.config/walker/omarchy/menu-extensions.sh to override the system menu with their own like adding the suspend function back.

#!/bin/bash

show_system_menu() {
  case $(menu "System" "  Lock\n󱄄  Screensaver\n󰒲  Suspend\n󰜉  Restart\n󰐥  Shutdown") in
  *Lock*) omarchy-lock-screen ;;
  *Screensaver*) omarchy-launch-screensaver force ;;
  *Suspend*) systemctl suspend ;;
  *Restart*) omarchy-cmd-reboot ;;
  *Shutdown*) omarchy-cmd-shutdown ;;
  *) back_to show_main_menu ;;
  esac
}

[Numilani]

For the mention: #3871

[dhh]

Great idea. Just changed the location to match the rest of the omarchy setup. We should consider other extension points like this.