feat(Core/Grids): Implement visibility notifier (#15919)

* Cherry-picked from TrinityCore (unable to find author)

src/server/game/Grids/Grid.h

@@ -96,6 +96,12 @@ public:
         visitor.Visit(i_objects);
     }
 
+    template<class T>
+    uint32 GetWorldObjectCountInGrid() const
+    {
+        return i_objects.template Count<T>();
+    }
+
     /** Inserts a container type object into the grid.
      */
     template<class SPECIFIC_OBJECT> void AddGridObject(SPECIFIC_OBJECT* obj)