Add `instance_as_child` method for PackedScene
Closes godotengine/godot-proposals#260.
As suggested by @bojidar-bg in https://github.com/godotengine/godot-proposals/issues/260#issuecomment-559285521.
Closes godotengine/godot-proposals#260.
As suggested by @bojidar-bg in https://github.com/godotengine/godot-proposals/issues/260#issuecomment-559285521.