Vehicle definitions moved to vehicles.json

This PR, similar to #2641, pulls the vehicle definitions out into a JSON file, getting rid of a huge block of macros.

"Wait a minute", you might say, "The vpart_id and vhtype_id enums are still in veh_type.h, so I still need to modify the code to add a vehicle!"

To which I can only respond "I know, and I'll refactor that later, but *veh_interact.cpp is a gigantic mess*."

For now, enjoy an externally-defined set of vehicle definitions - I'll get to figuring out what all the generic ints flying around in veh_interact.cpp are supposed to represent at a later date.