LibWeb: Implement AbortSignal.timeout()

This method returns a signal that will automatically abort after a
given number of milliseconds.

Tests/LibWeb/Text/input/abortsignal-timeout.html

@@ -0,0 +1,15 @@
+<script src="include.js"></script>
+<script>
+    asyncTest(done => {
+        const timeout_milliseconds = 10;
+        const test_start_time = performance.now();
+        const signal = AbortSignal.timeout(timeout_milliseconds);
+        signal.onabort = () => {
+            const abort_event_time = performance.now();
+            const time_taken_milliseconds = abort_event_time - test_start_time;
+            println(`Time passed before abort event fired is at least ${timeout_milliseconds} milliseconds: ${time_taken_milliseconds >= timeout_milliseconds}`);
+            println(`Reason type: ${signal.reason.name}`);
+            done();
+        };
+    });
+</script>