LibJS: Fix return statements not working properly in loops

Previously, when a loop detected an unwind of type ScopeType::Function
(which means a return statement was executed inside of the loop), it
would just return undefined. This set the VM's last_value to undefined,
when it should have been the returned value. This patch makes all loop
statements return the appropriate value in the above case.

Libraries/LibJS/Tests/return.js

@@ -0,0 +1,53 @@
+describe("returning from loops", () => {
+    test("returning from while loops", () => {
+        function foo() {
+            while (true) {
+                return 10;
+            }
+        }
+
+        expect(foo()).toBe(10);
+    });
+
+    test("returning from do-while loops", () => {
+        function foo() {
+            do {
+                return 10;
+            } while (true);
+        }
+
+        expect(foo()).toBe(10);
+    });
+
+    test("returning from for loops", () => {
+        function foo() {
+            for (let i = 0; i < 5; i++) {
+                return 10;
+            }
+        }
+
+        expect(foo()).toBe(10);
+    });
+
+    test("returning from for-in loops", () => {
+        function foo() {
+            const o = { a: 1, b: 2 };
+            for (let a in o) {
+                return 10;
+            }
+        }
+
+        expect(foo()).toBe(10);
+    });
+
+    test("returning from for-of loops", () => {
+        function foo() {
+            const o = [1, 2, 3];
+            for (let a of o) {
+                return 10;
+            }
+        }
+
+        expect(foo()).toBe(10);
+    });
+});