LibJS: Make prepare_temporal_fields() fields Object a const&

This only calls Object::get() on the fields Object, which is
const-qualified.

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

@@ -1104,7 +1104,7 @@ double to_positive_integer(GlobalObject& global_object, Value argument)
 }
 
 // 13.48 PrepareTemporalFields ( fields, fieldNames, requiredFields ), https://tc39.es/proposal-temporal/#sec-temporal-preparetemporalfields
-Object* prepare_temporal_fields(GlobalObject& global_object, Object& fields, Vector<String> const& field_names, Vector<StringView> const& required_fields)
+Object* prepare_temporal_fields(GlobalObject& global_object, Object const& fields, Vector<String> const& field_names, Vector<StringView> const& required_fields)
 {
     auto& vm = global_object.vm();
 

Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.h

@@ -112,7 +112,7 @@ Optional<TemporalTime> parse_temporal_time_string(GlobalObject&, String const& i
 Optional<TemporalTimeZone> parse_temporal_time_zone_string(GlobalObject&, String const& iso_string);
 Optional<TemporalYearMonth> parse_temporal_year_month_string(GlobalObject&, String const& iso_string);
 double to_positive_integer(GlobalObject&, Value argument);
-Object* prepare_temporal_fields(GlobalObject&, Object& fields, Vector<String> const& field_names, Vector<StringView> const& required_fields);
+Object* prepare_temporal_fields(GlobalObject&, Object const& fields, Vector<String> const& field_names, Vector<StringView> const& required_fields);
 
 // 13.46 ToIntegerThrowOnInfinity ( argument ), https://tc39.es/proposal-temporal/#sec-temporal-tointegerthrowoninfinity
 template<typename... Args>