LibJS: Fix \x escapes of bytes with high bit set

With this, typing `"\xff"` into Browser's console no longer
makes the app crash.

While here, also make the \u handler call append_codepoint()
instead of calling an overload where it's not immediately clear
which overload is getting called. This has no behavior change.

Libraries/LibJS/Tests/string-escapes.js

@@ -3,6 +3,7 @@ test("hex escapes", () => {
     expect("X55").toBe("X55");
     expect(`\x55`).toBe("U");
     expect(`\X55`).toBe("X55");
+    expect("\xff").toBe(String.fromCharCode(0xff));
 });
 
 test("unicode escapes", () => {
@@ -10,4 +11,5 @@ test("unicode escapes", () => {
     expect(`\u26a0`).toBe("⚠");
     expect("\u{1f41e}").toBe("🐞");
     expect(`\u{1f41e}`).toBe("🐞");
+    expect("\u00ff").toBe(String.fromCharCode(0xff));
 });