LibWeb: Move element_child_count to ParentNode and add its IDL attribute

I initially had it in Node just because, but then saw it was part of
ParentNode in the spec.

Userland/Libraries/LibWeb/DOM/Node.h

@@ -175,8 +175,6 @@ public:
 
     bool is_host_including_inclusive_ancestor_of(const Node&) const;
 
-    size_t element_child_count() const;
-
 protected:
     Node(Document&, NodeType);