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

Author: https://github.com/trflynn89
Commit: d2e63a641f
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.startsWith.js

@@ -34,3 +34,11 @@ test("basic functionality", () => {
     expect(s.startsWith("", -1)).toBeTrue();
     expect(s.startsWith("", 42)).toBeTrue();
 });
+
+test("UTF-16", () => {
+    var s = "😀";
+    expect(s.startsWith("😀")).toBeTrue();
+    expect(s.startsWith("\ud83d")).toBeTrue();
+    expect(s.startsWith("\ude00")).toBeFalse();
+    expect(s.startsWith("a")).toBeFalse();
+});