LibJS: Start implementing iterable framework, add ArrayIterator

With the addition of symbol keys, work can now be done on starting to
implement the well-known symbol functionality. The most important of
these well-known symbols is by far Symbol.iterator.

This patch adds IteratorPrototype, as well as ArrayIterator and
ArrayIteratorPrototype. In the future, sometime after StringIterator has
also been added, this will allow us to use Symbol.iterator directly in
for..of loops, enabling the use of custom iterator objects. Also makes
adding iterator support to native objects much easier (as will have to
be done for Map and Set, when they get added).

Libraries/LibJS/Tests/iterators/array-iterator.js

@@ -0,0 +1,48 @@
+test("length", () => {
+    expect(Array.prototype[Symbol.iterator].length).toBe(0);
+});
+
+test("same function as Array.prototype.values", () => {
+    expect(Array.prototype[Symbol.iterator]).toBe(Array.prototype.values);
+});
+
+test("basic functionality", () => {
+    const a = [1, 2, 3];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("works when applied to non-object", () => {
+    [true, false, 9, 2n, Symbol()].forEach(primitive => {
+        const it = [][Symbol.iterator].call(primitive);
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+    });
+});
+
+test("item added to array before exhaustion is accessible", () => {
+    const a = [1, 2];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    a.push(3);
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("item added to array after exhaustion is inaccesible", () => {
+    const a = [1, 2];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    a.push(3);
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});