Move mod vehicle registration to a decorator.

This cruft doesn't need to be in game.db, it can be kept with the mod
support code.

game/modsupport.py

@@ -0,0 +1,21 @@
+from typing import Type
+
+from dcs.helicopters import HelicopterType, helicopter_map
+from dcs.planes import PlaneType, plane_map
+from dcs.unittype import VehicleType
+from dcs.vehicles import vehicle_map
+
+
+def helicoptermod(helicopter: Type[HelicopterType]) -> Type[HelicopterType]:
+    helicopter_map[helicopter.id] = helicopter
+    return helicopter
+
+
+def planemod(plane: Type[PlaneType]) -> Type[PlaneType]:
+    plane_map[plane.id] = plane
+    return plane
+
+
+def vehiclemod(vehicle: Type[VehicleType]) -> Type[VehicleType]:
+    vehicle_map[vehicle.id] = vehicle
+    return vehicle