Turn off power supply off-load

Disable the high current output and wait a little before to turn off,
because the interrupting capacity of the PSU is unknown.
Could be a function if needed by other.

Marlin/Marlin_main.cpp

       #endif
       
       case 81: // M81 - Turn off Power Supply
-      
+        disable_heater();
+        st_synchronize();
+        disable_e0();
+        disable_e1();
+        disable_e2();
+        finishAndDisableSteppers();
+        fanSpeed = 0;
+        delay(1000); // Wait a little before to switch off
       #if defined SUICIDE_PIN && SUICIDE_PIN > -1
         st_synchronize();
         suicide();