Add possibility to set Z offset when creating instances from external layout

[AlexandreSi]

Fixes #5694

[AlexandreSi]

I feel like I should either set 0 as default value here or deprecate the method to have createObjectsFromExternalLayout2 because this method is exposed to JS events through gdjs.evtTools.runtimeScene.createObjectsFromExternalLayout.
Some JS events might use it and it would create bugs.

[4ian]

Alternative to createObjectsFrom2:

• At least, make the existing createObjectsFrom to call the new version (passing z = 0).
• Alternatively, do some "dynamic magic" by checking the value of arguments.length. If it's <= 4, treat zPos as trackByPersistentUuid with some ts-ignore.

So officially we support only the new version, but we have compatibility code to make existing calls still work

[AlexandreSi]

First time I use arguments! This is practical in that overloading case. I tried to see if I could declare the overloading with Typescript but I couldn't find a way.
After a bit of thinking, I think it's better not overloading the signature.

[4ian]

lgtm!