LibWeb: Implement AbortSignal.abort()

This returns an AbortSignal that is already set as aborted.

Tests/LibWeb/Text/expected/abortsignal-abort.txt

@@ -0,0 +1,4 @@
+Aborted: true
+Reason: "[object DOMException]"
+Aborted: true
+Reason: "This is a test"

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

@@ -0,0 +1,19 @@
+<script src="include.js"></script>
+<script>
+    asyncTest(async done => {
+        function testSignal(signal) {
+            return fetch("./basic.html", { signal })
+                .then(() => {
+                    println("FAIL");
+                })
+                .catch(error => {
+                    println(`Aborted: ${signal.aborted}`);
+                    println(`Reason: "${error}"`);
+                });
+        }
+
+        testSignal(AbortSignal.abort())
+            .then(() => testSignal(AbortSignal.abort("This is a test")))
+            .finally(done);
+    });
+</script>