LibJS: Implement the Intl.PluralRules constructor

Author: https://github.com/trflynn89
Commit: 348059bffd
Pull-request: https://github.com/SerenityOS/serenity/pull/12188
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/Intl/PluralRules.h

@@ -8,6 +8,7 @@
 
 #include <AK/String.h>
 #include <AK/StringView.h>
+#include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/Intl/NumberFormat.h>
 #include <LibJS/Runtime/Object.h>
 
@@ -33,4 +34,6 @@ private:
     Type m_type { Type::Cardinal }; // [[Type]]
 };
 
+ThrowCompletionOr<PluralRules*> initialize_plural_rules(GlobalObject& global_object, PluralRules& plural_rules, Value locales_value, Value options_value);
+
 }