Add Z Fade Height to M420 output if it is enabled (#7042)

* Add Z Fade Height to M420 Output

Echo the z-fade height when M420 is called, if it is enabled.

* Fix Whitespace

* Fix Compiler complaint

* Missing semi-colon

Marlin/Marlin_main.cpp

 
     SERIAL_ECHO_START();
     SERIAL_ECHOLNPAIR("Bed Leveling ", new_status ? MSG_ON : MSG_OFF);
+
+    #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
+      SERIAL_ECHO_START();
+      SERIAL_ECHOPGM("Fade Height ");
+      if (planner.z_fade_height > 0.0)
+        SERIAL_ECHOLN(planner.z_fade_height);
+      else
+        SERIAL_ECHOLNPGM(MSG_OFF);
+    #endif
   }
 #endif