Change exporting to "packs" instead of cars directly

[WhistleWiz]

This would change cars from being exported directly to instead being wrapped in a pack ScriptableObject. Doing so would decouple the mod info from being in the car itself to being completely separate, and would allow including extra files in a cleaner way.

The major reason for doing this change is to be able to include multiple cars in the same mod easily, while at the same time reducing the resources required. This is especially useful for steam locomotives as you could export the loco and the tender together, or for wagon packs that share resources (like WoTiger's Gmhs).

An example from the Car Changer mod: