LibJS: Make default_merge_fields() {additional_,}fields Object a const&

This only calls Object::{get,enumerable_own_property_names}() on the
fields and additional_fields Objects, both of which are const-qualified.

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

@@ -1034,7 +1034,7 @@ u8 iso_day(Object& temporal_object)
 }
 
 // 12.1.45 DefaultMergeFields ( fields, additionalFields ), https://tc39.es/proposal-temporal/#sec-temporal-defaultmergefields
-Object* default_merge_fields(GlobalObject& global_object, Object& fields, Object& additional_fields)
+Object* default_merge_fields(GlobalObject& global_object, Object const& fields, Object const& additional_fields)
 {
     auto& vm = global_object.vm();
 

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

@@ -72,6 +72,6 @@ i32 iso_year(Object& temporal_object);
 u8 iso_month(Object& temporal_object);
 String iso_month_code(Object& temporal_object);
 u8 iso_day(Object& temporal_object);
-Object* default_merge_fields(GlobalObject&, Object& fields, Object& additional_fields);
+Object* default_merge_fields(GlobalObject&, Object const& fields, Object const& additional_fields);
 
 }