LibJS: Implement Unicode aware String.prototype.to{Upper,Lower}Case

Author: https://github.com/trflynn89
Commit: 2e3a5b884c
Pull-request: https://github.com/SerenityOS/serenity/pull/9004
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toLowerCase.js

@@ -1,6 +1,10 @@
 test("basic functionality", () => {
     expect(String.prototype.toLowerCase).toHaveLength(0);
 
+    expect("ω".toLowerCase()).toBe("ω");
+    expect("Ω".toLowerCase()).toBe("ω");
+    expect("😀".toLowerCase()).toBe("😀");
+
     expect("foo".toLowerCase()).toBe("foo");
     expect("Foo".toLowerCase()).toBe("foo");
     expect("FOO".toLowerCase()).toBe("foo");

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toUpperCase.js

@@ -1,6 +1,10 @@
 test("basic functionality", () => {
     expect(String.prototype.toUpperCase).toHaveLength(0);
 
+    expect("ω".toUpperCase()).toBe("Ω");
+    expect("Ω".toUpperCase()).toBe("Ω");
+    expect("😀".toUpperCase()).toBe("😀");
+
     expect("foo".toUpperCase()).toBe("FOO");
     expect("Foo".toUpperCase()).toBe("FOO");
     expect("FOO".toUpperCase()).toBe("FOO");