4.6+ new self setup

since the `--script` launch parameter will be gone if the game doesn't use a custom compiled export template https://github.com/godotengine/godot/pull/111909 we need to change our setup process.

while override.cfg can be disabled completely now https://github.com/godotengine/godot/pull/108818 it is our only (non-intrusive) entry point for now.

we do however have the new `autoload_prepend` option for override.cfg https://github.com/godotengine/godot/pull/113078 . this means we can add an override which does not affect any other option in the project settings and only adds our autoloads. We don't necessarily need `--script` anymore since we don't need to read the project settings.

this means the new setup is probably just placing an override.cfg in the correct place (next to the `.exe` on windows/linux and inside the `.app` bundle `/Contents/MacOS/` folder). this mainly needs testing and a docs update.

i think with this prepend we won't need any direct injection with gdre or pcktool anymore.

for mac we might want to consider a tiny bash script to move the override in the correct location, since not that many people know about .app being a folder