LibJS: Ensure relevant extension keys are included in ICU locale data

This is a normative change in the ECMA-402 spec. See:
7508197

In our implementation, we don't have the affected AOs directly, as we
delegate to ICU. So instead, we must ensure we provide ICU a locale with
the relevant extension keys present.

Libraries/LibJS/Runtime/Intl/PluralRulesConstructor.cpp

@@ -87,8 +87,7 @@ ThrowCompletionOr<GC::Ref<Object>> PluralRulesConstructor::construct(FunctionObj
 
     // Non-standard, create an ICU number formatter for this Intl object.
     auto formatter = Unicode::NumberFormat::create(
-        plural_rules->locale(),
-        {},
+        result.icu_locale,
         {},
         plural_rules->rounding_options());