LibJS: Add the SetIterator built-in and Set.prototype.{values, entries}

While this implementation should be complete it is based on HashTable's
iterator, which currently follows bucket-order instead of the required
insertion order. This can be simply fixed by replacing the underlying
HashTable member in Set with an enhanced one that maintains a linked
list in insertion order.

Userland/Libraries/LibJS/Tests/builtins/Set/Set.prototype.values.js

@@ -0,0 +1,14 @@
+test("length", () => {
+    expect(Set.prototype.values.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const a = new Set([1, 2, 3]);
+    const it = a.values();
+    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 });
+});