Synchronously notify SceneTree before node is added

[janosdebugs]

This PR is a proof-of-concept implementation for godotengine/godot-proposals#8850. Its purpose is to allow for dependency injecting tree nodes.

Note: this is a PoC and a final implementation should check if the signals are processed synchronously or possibly a virtual function would be in order. Naming should also be verified.

Testing steps

1. Set up a standard Godot project and set up an autoload for a inject.gd file with the following contents:

class_name Injector extends Node

func _enter_tree() -> void:
	get_tree().node_adding.connect(_on_node_adding)

func _on_node_adding(node: Node) -> void:
	print("Adding node")

2. Create a main scene with the following attached script:

class_name Main extends Node2D

func _enter_tree() -> void:
	print("Enter tree")

func _ready() -> void:
	print("Ready")

Expected output:

Adding node
Enter tree
Ready

• Bugsquad edit, closes: Synchronous tree notification on child change godot-proposals#8850

[AThousandShips]

What is insufficient with the node_added signal? It's not mentioned here or in the proposal

[janosdebugs]

@AThousandShips updated the proposal to clarify.