Set has_mesh if user issues G9 S3

Prevents user from having to manually run through entire manual leveling process only to enable mesh so they can manually restore their offline mesh backup

Marlin/src/gcode/bedlevel/mbl/G29.cpp

         SERIAL_CHAR('Z'); echo_not_entered();
         return;
       }
+
+      mbl.has_mesh = true; // set since user manually entered a mesh point
+
       break;
 
     case MeshSetZOffset: