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.cpp

@@ -0,0 +1,11 @@
+#include <LibHTML/DOM/Comment.h>
+#include <LibHTML/Layout/LayoutText.h>
+
+Comment::Comment(Document& document, const String& data)
+    : CharacterData(document, NodeType::COMMENT_NODE, data)
+{
+}
+
+Comment::~Comment()
+{
+}