Basic Mob Brimdemon (#78424)

## About The Pull Request

Fixes #71330

The brimdemon was basically already perfect (well, it has a novel means
of attacking) so I didn't get too fancy with this one, it's _largely_
just a straightforward conversion.
Following this change it's a little slower to back off, but better at
lining up with people in order to blast them. Additionally, its beam is
now a mob ability so you can give it to other mobs if you so desire.

Because I can't help doing a _little_ tinkering, Brimdemons now explode
2.5 seconds after they die, after a brief warning animation.

## Why It's Good For The Game

Simple mobs must die

## Changelog

🆑
add: Brimdemon corpses release an explosion shortly after death, just to
keep you on your toes.
refactor: Brimdemons now use the basic mob framework which (should)
improve their pathfinding somewhat. Please bug report any unusual
behaviour.
admin: The brimdemon's beam ability can be given to any mob, for your
Binding of Isaac event
/🆑

code/datums/ai/basic_mobs/basic_subtrees/flee_target.dm

@@ -9,10 +9,7 @@
 
 /datum/ai_planning_subtree/flee_target/SelectBehaviors(datum/ai_controller/controller, seconds_per_tick)
 	. = ..()
-	if (!controller.blackboard[BB_BASIC_MOB_FLEEING])
-		return
-	var/atom/target = controller.blackboard[target_key]
-	if(QDELETED(target))
+	if (!controller.blackboard[BB_BASIC_MOB_FLEEING] || !controller.blackboard_key_exists(target_key))
 		return
 	controller.queue_behavior(flee_behaviour, target_key, hiding_place_key)
 	return SUBTREE_RETURN_FINISH_PLANNING //we gotta get out of here.