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

Author: https://github.com/trflynn89
Commit: 60d8852fc2
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.substring.js

@@ -12,3 +12,14 @@ test("basic functionality", () => {
     expect("hello friends".substring(0, "5")).toBe("hello");
     expect("hello friends".substring("6", "13")).toBe("friends");
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s).toHaveLength(2);
+    expect(s.substring()).toBe("😀");
+    expect(s.substring(0)).toBe("😀");
+    expect(s.substring(0, 2)).toBe("😀");
+    expect(s.substring(0, 1)).toBe("\ud83d");
+    expect(s.substring(1, 2)).toBe("\ude00");
+    expect(s.substring(2, 2)).toBe("");
+});