Configuration option for number of roundstart mice (#26133)

This was actually requested, because it turns out testing powernets is
hard if mice randomly chew wires sometimes.

code/controllers/subsystem/squeak.dm

@@ -10,9 +10,11 @@ SUBSYSTEM_DEF(squeak)
 	var/list/exposed_wires = list()
 
 /datum/controller/subsystem/squeak/Initialize(timeofday)
-	trigger_migration()
+	trigger_migration(config.mice_roundstart)
 
 /datum/controller/subsystem/squeak/proc/trigger_migration(num_mice=10)
+	if(!num_mice)
+		return
 	find_exposed_wires()
 
 	var/mob/living/simple_animal/mouse/M