Fixes overmap vehicle tracking/remembering with bike rack

[SeventhSandwich]

SUMMARY: Bugfixes "Fixes overmap vehicle tracking/remembering with bike rack"

Purpose of change

Closes #37311
When the player chooses to 'Remember Vehicle Position', the overmap tracking is lost when the vehicle is loaded and unloaded on a bike rack. This leads to annoying situations where a tracked shopping cart gets lost after unloading, without the player ever having chosen to forget it.

Describe the solution

This PR incorporates a new vehicle part integer flag called 'tracked_flag' that is assigned when a vehicle with tracking turned on is carried on a bike rack. When the bike rack is unloaded, the vehicle is added back to the overmap for tracking.

The way it works is that a tracked-vehicle loaded on a bike rack stops being tracked until it's unloaded, and then tracking is automatically enabled.

Describe alternatives you've considered

I considered adding functionality to track an individual vehicle part on the overmap - to allow racked vehicles to be tracked on the map while being carried. That seemed extraneous for just one bugfix.

Testing

I built CDDA in Visual Studio 2019 and tested various combinations of racking one or multiple vehicles with/without tracking them. My changes seem to work as expected.

Note that unloading a tracked vehicle sends the same message to the player as turning on tracking normally. "You start keeping track of this vehicle's position". I don't consider this a problem.

Additional context

None