LibWeb: Make ParentNode.children vend the same HTMLCollection every time

Instead of creating a new HTMLCollection every time you access
.children, we now follow the spec and vend the same object.

This was annoyingly difficult before, and trivial now that the DOM is
garbage-collected. :^)

Userland/Libraries/LibWeb/DOM/ParentNode.h

@@ -45,6 +45,11 @@ protected:
         : Node(document, type)
     {
     }
+
+    virtual void visit_edges(Cell::Visitor&) override;
+
+private:
+    JS::GCPtr<HTMLCollection> m_children;
 };
 
 template<>