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

Author: https://github.com/trflynn89
Commit: f920e121b3
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.lastIndexOf.js

@@ -20,3 +20,11 @@ test("basic functionality", () => {
     expect("hello friends serenity".lastIndexOf("s", 13)).toBe(12);
     expect("hello".lastIndexOf("serenity")).toBe(-1);
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s.lastIndexOf("😀")).toBe(0);
+    expect(s.lastIndexOf("\ud83d")).toBe(0);
+    expect(s.lastIndexOf("\ude00")).toBe(1);
+    expect(s.lastIndexOf("a")).toBe(-1);
+});