implement override.cfg autoload order manipulation

[Qubus0]

it seems to be possible to place the mod loader autoloads first in order by setting all vanilla autoloads to null. this requires manual file editing though, since ConfigFile doesn't allow setting to null (it will remove the key instead) afaik

example

[autoload]
VanillaAutoload=null
ModLoader=*res://addons/mod_loader.gd
VanillaAutoload=*res://original/path/vanilla_autoload.gd

ref godotengine/godot-proposals#6137 (comment)