LibJS: Add an Iterator constructor and object

The Iterator object cannot be constructed directly but can be subclassed
or created with `Iterator.from` (not implemented here).

Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.js

@@ -0,0 +1,27 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Iterator();
+        }).toThrowWithMessage(TypeError, "Iterator constructor must be called with 'new'");
+    });
+
+    test("cannot be directly constructed", () => {
+        expect(() => {
+            new Iterator();
+        }).toThrowWithMessage(TypeError, "Abstract class Iterator cannot be constructed directly");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(Iterator).toHaveLength(0);
+    });
+
+    test("can be constructed from with subclass", () => {
+        class TestIterator extends Iterator {}
+
+        const iterator = new TestIterator();
+        expect(iterator).toBeInstanceOf(TestIterator);
+        expect(iterator).toBeInstanceOf(Iterator);
+    });
+});