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/Comment.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <AK/String.h>
+#include <LibHTML/DOM/CharacterData.h>
+
+class Comment final : public CharacterData {
+public:
+    explicit Comment(Document&, const String&);
+    virtual ~Comment() override;
+
+    virtual String tag_name() const override { return "#comment"; }
+};
+
+template<>
+inline bool is<Comment>(const Node& node)
+{
+    return node.is_comment();
+}