LibWeb: Add CDATASection

Not used for anything currently other than exposing it on the Window
object. This allows Web Components Polyfills to patch its prototype.

Userland/Libraries/LibWeb/Forward.h

@@ -118,6 +118,7 @@ class AbstractRange;
 class AbortController;
 class AbortSignal;
 class Attribute;
+class CDATASection;
 class CharacterData;
 class Comment;
 class CustomEvent;
@@ -410,6 +411,7 @@ class AttributeWrapper;
 struct CallbackType;
 class CanvasGradientWrapper;
 class CanvasRenderingContext2DWrapper;
+class CDATASectionWrapper;
 class CharacterDataWrapper;
 class CloseEventWrapper;
 class CommentWrapper;