Starts chase mode for the given slasher.
Unless forceChase is used, this looks for a survivor in the slasher's eye trace or view cone within their SLASHER.ChaseRange, and does nothing if none is found or chasing is disabled for them.
If the slasher is already chasing, this calls SlashCo.StopChase instead.
On success it sets InSlasherChaseMode, switches to SLASHER.ChaseSpeed, plays the chase music, adds the SlasherChase fog, and schedules an automatic SlashCo.StopChase after the chase duration.