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

Author: https://github.com/trflynn89
Commit: 5d11614bc7
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.charAt.js

@@ -18,3 +18,11 @@ test("basic functionality", () => {
     expect(s.charAt("foo")).toBe("f");
     expect(s.charAt(undefined)).toBe("f");
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s).toHaveLength(2);
+    expect(s.charAt(0)).toBe("\ud83d");
+    expect(s.charAt(1)).toBe("\ude00");
+    expect(s.charAt(2)).toBe("");
+});