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/Text.cpp

@@ -2,8 +2,7 @@
 #include <LibHTML/Layout/LayoutText.h>
 
 Text::Text(Document& document, const String& data)
-    : Node(document, NodeType::TEXT_NODE)
-    , m_data(data)
+    : CharacterData(document, NodeType::TEXT_NODE, data)
 {
 }