Add a method to convert a scene to an instanced scene

[TrickMe]

Describe the project you are working on

Realtime space x4 with procedurally generated galaxy.

Describe the problem or limitation you are having in your project

I noticed that it is faster to procedurally generate a game scene, while the scene is not in the tree.
So I generate the world out of tree and then add the main node to the tree.

get_tree().change_scene() and get_tree().change_scene_to() only accept PackedScenes, not nodes.
(or instanced PackedScenes, with nodes like sprites, etc. procedurally added to them)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

solutions:
1
make get_tree().change_scene() and get_tree().change_scene_to() accept instanced PackedScenes instead of PackedScenes

2
add get_tree().change_scene_instance() and get_tree().change_scene_to_instance() methods.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

solution 1:
get_tree().change_scene(newScene.instance()) instead of get_tree().change_scene(newScene)
get_tree().change_scene_to(newScene.instance()) instead of get_tree().change_scene_to(newScene)

If this enhancement will not be used often, can it be worked around with a few lines of script?

add onready var and func to Autoload "GLOBAL":

onready var currentScene:Node = get_node("/root/firstLoadedSceneName")

func change_scene(newScene:Node):
    currentScene.queue_free()
    currentScene = newScene
    get_tree().get_root().add_child(newScene)

use in script:

GLOBAL.change_scene(PackedScene.instance())

Is there a reason why this should be core and not an add-on in the asset library?

Although there is an easy workaround, I think it could be beneficial for users and beginners, who generate their game world procedurally.
The change in solution 1 is only to add .instance() to the script to update your project to work.

edit:
If a scene is added to the tree, it always has to be instanced first, change_scene() seems to be the only exception
So for consistency I encourage solution 1.

[Calinou]

See also #2367.

[TrickMe]

Isn't the method to convert a scene to an instanced scene literally .instance()?
I'm worrying if the new headline could be misinterpreted.

[KoBeWi]

Duplicate of #921 (former godotengine/godot#24795)