LibJS: Update Object::define_accessor() to take both getter and setter

This replaces the current 'function plus boolean indicating the type'
API, which makes it easier to set both getter and setter at once.
This was already possible before but required two calls of this
function, which wasn't intuitive:

    define_accessor(name, getter, true, ...);
    define_accessor(name, setter, false, ...);

Which now becomes:

    define_accessor(name, getter, setter, ...);

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -536,7 +536,7 @@ bool Object::define_property(const PropertyName& property_name, Value value, Pro
     return put_own_property(property_name.to_string_or_symbol(), value, attributes, PutOwnPropertyMode::DefineProperty, throw_exceptions);
 }
 
-bool Object::define_accessor(const PropertyName& property_name, Function& getter_or_setter, bool is_getter, PropertyAttributes attributes, bool throw_exceptions)
+bool Object::define_accessor(const PropertyName& property_name, Function* getter, Function* setter, PropertyAttributes attributes, bool throw_exceptions)
 {
     VERIFY(property_name.is_valid());
 
@@ -548,18 +548,18 @@ bool Object::define_accessor(const PropertyName& property_name, Function& getter
             accessor = &existing_property.as_accessor();
     }
     if (!accessor) {
-        accessor = Accessor::create(vm(), nullptr, nullptr);
+        accessor = Accessor::create(vm(), getter, setter);
         bool definition_success = define_property(property_name, accessor, attributes, throw_exceptions);
         if (vm().exception())
             return {};
         if (!definition_success)
             return false;
+    } else {
+        if (getter)
+            accessor->set_getter(getter);
+        if (setter)
+            accessor->set_setter(setter);
     }
-    if (is_getter)
-        accessor->set_getter(&getter_or_setter);
-    else
-        accessor->set_setter(&getter_or_setter);
-
     return true;
 }