Add DUPLICATE_INTERNAL_STATE flag

[KoBeWi]

Resolves #3393

[YuriSizov]

Is this still needed? @kubecz3k doesn't seem to recall if it is 🙃

[KoBeWi]

Well, internal (non-exported) properties are still not duplicated, soo...

[kubecz3k]

So I updated the original issue by converting reproduction project from Godot (1.x?) to Godot 4. I can confirm the issue is still the same.

Also to clarify: the issue is about usage of Object.duplicate() method and the fact that variables of returned object (even simple types) are different from the object from which it has been duplicated

Leaved some additional comment in the issue #3393 (comment)

[akien-mga]

I don't understand why validation still fails:

Validate extension JSON: Error: Hash changed for 'classes/Node/methods/duplicate', from D14E1583 to A4AD8DB9. This means that the function has changed and no compatibility function was provided.

The compat binding seems correct to me.

Maybe because you didn't change the C++ prototype of Node::duplicate itself to add the new default flag? Though I don't see why it would be involved here.

[KoBeWi]

Ok I reworked the changes a bit. The PR no longer breaks compatibility, but the new flag is not enabled by default. Instead I improved the documentation and added a new DUPLICATE_DEFAULT flag to make the new flag easier to use.

[Repiteo]

Thanks!