Update for new NOTIFICATION_POSTINITIALIZE handling

[dsnopek]

This adds support for the Godot changes from PR https://github.com/godotengine/godot/pull/91018

With this change, we should finally be handling NOTIFICATION_POSTINITIALIZE in a way that matches how it works in Godot itself, which should allow (among other things) truly internal child nodes.

@Daylily-Zeleen If you have time, could you test that this works in your project?

Fixes https://github.com/godotengine/godot-cpp/issues/1567

[Daylily-Zeleen]

Test this commit with MRP in this issue and v4.4.dev.custom_build [61598c5c8], not crsh occur.

[dsnopek]

@Daylily-Zeleen I tested duplicating "MyNode" in your MRP, and it's working for me!

We still need someone who is not me (as the author of this PR) to review and approve it, though :-)