LibHTML: Add Comment and CharacterData nodes and improve HTML parsing

This patch adds the CharacterData subclass of Node, which is now the
parent class of Text and a new Comment class.

A Comment node is one of these in HTML: <!--hello friends-->
Since these occur somewhat frequently on the web, we need to be able
to parse them.

This patch also adds a child rejection mechanism to the DOM tree.
Nodes can now override is_child_allowed(Node) and return false if they
don't want a particular Node to become a child of theirs. This is used
to prevent Document from taking on unwanted children.

Libraries/LibHTML/DOM/CharacterData.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <AK/String.h>
+#include <LibHTML/DOM/Node.h>
+
+class CharacterData : public Node {
+public:
+    virtual ~CharacterData() override;
+
+    const String& data() const { return m_data; }
+
+    virtual String text_content() const override { return m_data; }
+
+protected:
+    explicit CharacterData(Document&, NodeType, const String&);
+
+private:
+    String m_data;
+};
+
+template<>
+inline bool is<CharacterData>(const Node& node)
+{
+    return node.is_character_data();
+}