refactor: extract to vehicle_part.h

[scarf005]

Summary

SUMMARY: Infrastructure "Separate vehicle.h with vehicle_part.h"

• blocked by perf(port): various debug.cpp improvements #3101 (will rebase once it gets approved and merged)

Purpose of change

vehicle.cpp and vehicle_part.cpp are already separated, doing the same for headers make sense as it means less chance to recompile everything (in the future)

Describe the solution

cut and pasted vehicle_part class into vehicle_part.h.

Testing

built without errors on my machine.

[olanti-p]

Looks like many .cpp files don't necessarily need including vehicle_part.h, but that can be fixed later by iwyu.