Implement Node custom iterator for traversing children

[Xrayez]

This PR is created as a minimal example implementation to demonstrate performance issues with using custom iterators in C++ as described in #42053.

Allows to iterate children with a plain for loop without using Node.get_children():

extends Node2D

func _ready():
	for child in $node:
		print(child)

Unfortunately, performs 10 times worse than allocating an array from get_children() and iterating it (even with 100000 nodes).

On a side note, I think this could be improved to the point that using a built-in custom iterator in C++ (#7225) could prove to be faster than current way of traversing children with get_children(), and this could be potentially in alignment to how it's currently possible to for i in number without allocating array for similar reasons, and obviously this would apply to any other custom data structure, such as linked list.

[Xrayez]

This PR was created to demonstrate performance issues with #42053, so closing, you'll still be able to compile the PR if you need to test this.