LibJS: Implement RegExp.prototype [ @@matchAll ]

This also allows String.prototype.matchAll to work, as all calls to that
method result in an invocation to @@matchAll.

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.matchAll.js

@@ -0,0 +1,78 @@
+test("invariants", () => {
+    expect(String.prototype.matchAll).toHaveLength(1);
+});
+
+test("error cases", () => {
+    [null, undefined].forEach(value => {
+        expect(() => {
+            value.matchAll("");
+        }).toThrow(TypeError);
+    });
+
+    expect(() => {
+        "hello friends".matchAll(/hello/);
+    }).toThrow(TypeError);
+});
+
+test("basic functionality", () => {
+    expect("hello friends".matchAll(/hello/g)).not.toBeNull();
+    expect("hello friends".matchAll(/enemies/g)).not.toBeNull();
+
+    {
+        var iterator = "".matchAll(/a/g);
+
+        var next = iterator.next();
+        expect(next.done).toBeTrue();
+        expect(next.value).toBeUndefined();
+
+        next = iterator.next();
+        expect(next.done).toBeTrue();
+        expect(next.value).toBeUndefined();
+    }
+    {
+        var iterator = "a".matchAll(/a/g);
+
+        var next = iterator.next();
+        expect(next.done).toBeFalse();
+        expect(next.value).toEqual(["a"]);
+        expect(next.value.index).toBe(0);
+
+        next = iterator.next();
+        expect(next.done).toBeTrue();
+        expect(next.value).toBeUndefined();
+    }
+    {
+        var iterator = "aa".matchAll(/a/g);
+
+        var next = iterator.next();
+        expect(next.done).toBeFalse();
+        expect(next.value).toEqual(["a"]);
+        expect(next.value.index).toBe(0);
+
+        next = iterator.next();
+        expect(next.done).toBeFalse();
+        expect(next.value).toEqual(["a"]);
+        expect(next.value.index).toBe(1);
+
+        next = iterator.next();
+        expect(next.done).toBeTrue();
+        expect(next.value).toBeUndefined();
+    }
+    {
+        var iterator = "aba".matchAll(/a/g);
+
+        var next = iterator.next();
+        expect(next.done).toBeFalse();
+        expect(next.value).toEqual(["a"]);
+        expect(next.value.index).toBe(0);
+
+        next = iterator.next();
+        expect(next.done).toBeFalse();
+        expect(next.value).toEqual(["a"]);
+        expect(next.value.index).toBe(2);
+
+        next = iterator.next();
+        expect(next.done).toBeTrue();
+        expect(next.value).toBeUndefined();
+    }
+});