LibHTML: Templatize Node::first_child_of_type<T>()

This is a lot nicer than first_child_with_tag_name(...).

The is<T>(Node) functions are obviously unoptimized at the moment,
and this is about establishing pleasant patterns right now. :^)

Libraries/LibHTML/DOM/HTMLTitleElement.h

@@ -7,3 +7,9 @@ public:
     HTMLTitleElement(Document&, const String& tag_name);
     virtual ~HTMLTitleElement() override;
 };
+
+template<>
+inline bool is<HTMLTitleElement>(const Node& node)
+{
+    return is<Element>(node) && to<Element>(node).tag_name().to_lowercase() == "title";
+}