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/Tests/DOM/AbortController.js

@@ -0,0 +1,20 @@
+describe("AbortController", () => {
+    loadLocalPage("/res/html/misc/blank.html");
+
+    afterInitialPageLoad(page => {
+        test("Basic functionality", () => {
+            const abortController = new page.AbortController();
+            let timesCallbackCalled = 0;
+            abortController.signal.addEventListener("abort", () => {
+                timesCallbackCalled++;
+            });
+
+            abortController.abort();
+            expect(abortController.signal.aborted).toBeTrue();
+
+            abortController.abort();
+            expect(timesCallbackCalled).toBe(1);
+        });
+    });
+    waitForPageToLoad();
+});