LibHTML: Add DocumentFragment

Right now this is just a simple ParentNode subclass with its node type
being DOCUMENT_FRAGMENT_NODE.

Libraries/LibHTML/DOM/DocumentFragment.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <LibHTML/DOM/ParentNode.h>
+
+class DocumentFragment : public ParentNode {
+public:
+    DocumentFragment(Document& document)
+        : ParentNode(document, NodeType::DOCUMENT_FRAGMENT_NODE)
+    {
+    }
+
+    virtual String tag_name() const override { return "#document-fragment"; }
+};
+
+template<>
+inline bool is<DocumentFragment>(const Node& node)
+{
+    return node.is_document_fragment();
+}