LibWeb: Add initial support for AbortController and AbortSignal

The DOM specification says that the primary use case for these is to
give Promises abort semantics. It is also a prerequisite for Fetch,
as it is used to make Fetch abortable.
a

Userland/Libraries/LibWeb/Forward.h

@@ -32,6 +32,8 @@ enum class Display;
 }
 
 namespace Web::DOM {
+class AbortController;
+class AbortSignal;
 class CharacterData;
 class Comment;
 class Document;
@@ -203,6 +205,8 @@ class XMLHttpRequestEventTarget;
 }
 
 namespace Web::Bindings {
+class AbortControllerWrapper;
+class AbortSignalWrapper;
 class CSSStyleDeclarationWrapper;
 class CSSStyleSheetWrapper;
 class CanvasRenderingContext2DWrapper;