M110 is a known command

- The command parser now displays “unknown command” in more cases.
Known commands must be added to the `switch` inside `process_commands`
to suppress the error.

Marlin/Marlin_main.cpp

         gcode_M109();
         break;
 
+      case 110: break; // M110: Set line number - don't show "unknown command"
+
       #if HAS_TEMP_BED
         case 190: // M190: Wait for bed heater to reach target
           gcode_M190();