LibHTML: Add is<ElementType> and to<ElementType> helper functions

These will help us write node-type-aware template functions.

Libraries/LibHTML/DOM/HTMLHtmlElement.h

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