Add MMU2 capability line

Firmware/Marlin_main.cpp

@@ -4060,6 +4060,7 @@ static void extended_capabilities_report()
     cap_line(PSTR("AUTOREPORT_POSITION"), ENABLED(AUTO_REPORT));
     // EXTENDED_M20 (support for L and T parameters)
     cap_line(PSTR("EXTENDED_M20"), 1);
+    cap_line(PSTR("PRUSA_MMU2"), 1); //this will soon change to ENABLED(PRUSA_MMU2_SUPPORT)
 }
 #endif //EXTENDED_CAPABILITIES_REPORT