Utilities to work with the node tree

[Bromeon]

Similar to the old NodeResolveExt, utility methods would allow more idiomatic access to the scene tree. Those could be provided as an extension trait or in a dedicated module/struct.

Examples:

• [ ] get_node<T>
• [ ] find_node<T> (optional)
• [ ] load<T>
• [ ] PackedScene::instance<T>