LibJS: Add the MapIterator built-in and the key/values/entries methods

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

Userland/Libraries/LibJS/Tests/builtins/Map/Map.prototype.entries.js

@@ -0,0 +1,26 @@
+test("length", () => {
+    expect(Map.prototype.entries.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const original = [
+        ["a", 0],
+        ["b", 1],
+        ["c", 2],
+    ];
+    const a = new Map(original);
+    const it = a.entries();
+    // FIXME: This test should be rewritten once we have proper iteration order
+    const first = it.next();
+    expect(first.done).toBeFalse();
+    expect(a.has(first.value[0])).toBeTrue();
+    const second = it.next();
+    expect(second.done).toBeFalse();
+    expect(a.has(second.value[0])).toBeTrue();
+    const third = it.next();
+    expect(third.done).toBeFalse();
+    expect(a.has(third.value[0])).toBeTrue();
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});

Userland/Libraries/LibJS/Tests/builtins/Map/Map.prototype.keys.js

@@ -0,0 +1,26 @@
+test("length", () => {
+    expect(Map.prototype.keys.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const original = [
+        ["a", 0],
+        ["b", 1],
+        ["c", 2],
+    ];
+    const a = new Map(original);
+    const it = a.keys();
+    // FIXME: This test should be rewritten once we have proper iteration order
+    const first = it.next();
+    expect(first.done).toBeFalse();
+    expect(a.has(first.value)).toBeTrue();
+    const second = it.next();
+    expect(second.done).toBeFalse();
+    expect(a.has(second.value)).toBeTrue();
+    const third = it.next();
+    expect(third.done).toBeFalse();
+    expect(a.has(third.value)).toBeTrue();
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});

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

@@ -0,0 +1,26 @@
+test("length", () => {
+    expect(Map.prototype.values.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const original = [
+        ["a", 0],
+        ["b", 1],
+        ["c", 2],
+    ];
+    const a = new Map(original);
+    const it = a.values();
+    // FIXME: This test should be rewritten once we have proper iteration order
+    const first = it.next();
+    expect(first.done).toBeFalse();
+    expect([0, 1, 2].includes(first.value)).toBeTrue();
+    const second = it.next();
+    expect(second.done).toBeFalse();
+    expect([0, 1, 2].includes(second.value)).toBeTrue();
+    const third = it.next();
+    expect(third.done).toBeFalse();
+    expect([0, 1, 2].includes(third.value)).toBeTrue();
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});