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/DOM/AbortSignal.idl

@@ -0,0 +1,8 @@
+[Exposed=(Window,Worker)]
+interface AbortSignal : EventTarget {
+    // FIXME: [NewObject] static AbortSignal abort();
+
+    readonly attribute boolean aborted;
+
+    // FIXME: attribute EventHandler onabort;
+};