SimpleSpawner: Allow spawning nodes as child of scene

In the current implementation, spawned nodes are children of the spawner. In
some cases this behaviour is what you want, but it means it's not possible to
change the spawn position of future scenes without moving already-spawned
scenes.

Add a new property to SimpleSpawner which allows choosing a new mode where
spawned scenes are made children of the root scene.

The change in spawn_start() is necessary because, if the Start Spawning block
is used in the On Ready entrypoint, this results in spawn_start() being called
from the SimpleSpawner's _ready() function. At this point, the SimpleSpawner
itself is ready but the root scene is not. In this case, attempting to add a
child to the root scene logs an error, and a later attempt to get the spawned
scene's parent to remove it returns undefined.

Author: wjt

addons/block_code/simple_spawner/simple_spawner.gd

@@ -7,11 +7,20 @@ const BlocksCatalog = preload("res://addons/block_code/code_generation/blocks_ca
 const OptionData = preload("res://addons/block_code/code_generation/option_data.gd")
 const Types = preload("res://addons/block_code/types/types.gd")
 
+enum SpawnParent {
+	THIS,  ## Spawned scenes are children of this node
+	SCENE,  ## Spawned scenes are children of the scene
+}
 enum LimitBehavior { REPLACE, NO_SPAWN }
 
 ## The scenes to spawn. If more than one are provided, they will be picked randomly.
 @export var scenes: Array[PackedScene] = []
 
+## The node that the spawned scenes should be a child of. If you want to move
+## the SimpleSpawner without moving the scenes it has already spawned, choose
+## SCENE.
+@export var spawn_parent: SpawnParent
+
 ## The period of time in seconds to spawn another component. If zero, they won't spawn
 ## automatically. Use the "Spawn" block.
 @export_range(0.0, 10.0, 0.1, "or_greater") var spawn_frequency: float = 0.0:
@@ -41,7 +50,7 @@ func get_custom_class():
 func _remove_oldest_spawned():
 	var spawned = _spawned_scenes.pop_front()
 	if is_instance_valid(spawned):
-		remove_child(spawned)
+		spawned.get_parent().remove_child(spawned)
 
 
 func _set_spawn_fraquency(new_frequency: float):
@@ -60,7 +69,7 @@ func spawn_start():
 		_timer.wait_time = spawn_frequency
 		_timer.timeout.connect(spawn_once)
 	_timer.start()
-	spawn_once()
+	spawn_once.call_deferred()
 
 
 func spawn_stop():
@@ -90,7 +99,12 @@ func spawn_once():
 	var scene: PackedScene = scenes.pick_random()
 	var spawned = scene.instantiate()
 	_spawned_scenes.push_back(spawned)
-	add_child(spawned)
+	match spawn_parent:
+		SpawnParent.THIS:
+			add_child(spawned)
+		SpawnParent.SCENE:
+			get_tree().current_scene.add_child(spawned)
+			spawned.position = global_position
 
 
 func do_set_spawn_frequency(new_frequency: float):