Deprecate Resource.setup_local_to_scene

[Mickeon]

This PR has been updated to deprecate instead of removing. Compatibility is no longer broken because the method still works, but use is discouraged.

Continuation of #67080 and in a way, #67072.

To quote myself from Godot's RocketChat:

What? Why? I can't even find the reasoning as to why this happened, nor the reasoning as to why setup_local_to_scene is exposed to users in the first place! It was so difficult to explain in my documentation tweak. It does not feel right. Custom resources that would need this are expected to run their own specialised logic, their own functions, written by the more advanced user that knows what they're doing.

Above, I mentioned that it was a struggle to explain properly. And that is because it currently does something really trivial: emit the setup_local_to_scene_requested to potentially perform custom user logic. But internally, it is expected to be called only once during PackedScene.instantiate().

Because the user may also call it whenever they want, they need to understand the occasion the engine expects it to be called. Specifically, deserialization of a local_to_scene resource. But again, if the user is writing custom serialization for a Resource for whatever reason, they will write their own methods to do it, without ever calling setup_local_to_scene!

In conclusion, good riddance, please! ... See you in Godot 5...!

[Mickeon]

Rebased. Bumped.

[KoBeWi]

Makes sense, but the method can't be removed, because it breaks compatibility. You can deprecate it and make no-op.

[akien-mga]

Makes sense, but the method can't be removed, because it breaks compatibility. You can deprecate it and make no-op.

As a reminder, deprecating means that the functionality still exists, but may be removed in the future.

If it's made no-op, there's no point in keeping it, as it breaks compatibility anyway (breaks projects that use it).

[Mickeon]

As edited above:

This PR has been updated to deprecate instead of removing. Compatibility is no longer broken because the method still works, but use is discouraged.

[Mickeon]

Merged too soon? I suppose the description can be changed on the other PR.

[akien-mga]

Yeah I forgot to remove it from my merge queue after commenting :)