A general tidy-up for vehicle code to make things easier in the future.
- Split BaseVehicle into BaseVehicle, BaseVehicle.Server, and BaseVehicle.Client.
- Move IsWaterlogged and CanRunEngine checks to VehicleEngineController, so they can be shared by everything with an engine.
- Match EntityFuelSystem and VehicleEngineController to Rust code formatting ('private' written out, spaces inside brackets).
- BaseVehicle IsMounted implementation fix.