LibJS: Implement ECMA-402 String.prototype.toLocale{Lower,Upper}Case

Author: https://github.com/trflynn89
Commit: 207319ecf1
Pull-request: https://github.com/SerenityOS/serenity/pull/9841
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp

@@ -233,7 +233,7 @@ Vector<String> canonicalize_locale_list(GlobalObject& global_object, Value local
 }
 
 // 9.2.2 BestAvailableLocale ( availableLocales, locale ), https://tc39.es/ecma402/#sec-bestavailablelocale
-static Optional<String> best_available_locale(StringView const& locale)
+Optional<String> best_available_locale(StringView const& locale)
 {
     // 1. Let candidate be locale.
     StringView candidate = locale;