🎨 Add MMU2 enabled() accessor

Marlin/src/feature/mmu/mmu2.h

@@ -43,6 +43,7 @@ public:
 
   static void init();
   static void reset();
+  static inline bool enabled() { return _enabled; }
   static void mmu_loop();
   static void tool_change(const uint8_t index);
   static void tool_change(const char *special);
@@ -88,7 +89,7 @@ private:
     static void mmu_continue_loading();
   #endif
 
-  static bool enabled, ready, mmu_print_saved;
+  static bool _enabled, ready, mmu_print_saved;
 
   static uint8_t cmd, cmd_arg, last_cmd, extruder;
   static int8_t state;