LibJS: Implement String.prototype.at with UTF-16 code units

Author: https://github.com/trflynn89
Commit: ef2ff5f88b
Pull-request: https://github.com/SerenityOS/serenity/pull/8897
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/davidot

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

@@ -13,3 +13,11 @@ test("basic functionality", () => {
     expect(string.at(-4)).toBeUndefined();
     expect(string.at(-Infinity)).toBeUndefined();
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s).toHaveLength(2);
+    expect(s.at(0)).toBe("\ud83d");
+    expect(s.at(1)).toBe("\ude00");
+    expect(s.at(2)).toBeUndefined();
+});