Add kill action

Sends pre-defined string as action command when printer is kill.

Marlin/Marlin_main.cpp

@@ -12836,6 +12836,10 @@ void kill(const char* lcd_msg) {
   _delay_ms(250); //Wait to ensure all interrupts routines stopped
   thermalManager.disable_all_heaters(); //turn off heaters again
 
+  #if defined(ACTION_ON_KILL)
+    SERIAL_ECHOLNPGM("//action:" ACTION_ON_KILL);
+  #endif
+  
   #if HAS_POWER_SWITCH
     SET_INPUT(PS_ON_PIN);
   #endif