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/game/objects/effects/spawners/random/lavaland_mobs.dm

@@ -7,11 +7,11 @@
 	icon_state = "large_egg"
 	loot = list(
 		/mob/living/basic/mining/bileworm = 1,
+		/mob/living/basic/mining/brimdemon = 1,
 		/mob/living/basic/mining/goldgrub = 1,
 		/mob/living/basic/mining/goliath = 1,
 		/mob/living/basic/mining/lobstrosity/lava = 1,
 		/mob/living/basic/mining/watcher = 1,
-		/mob/living/simple_animal/hostile/asteroid/brimdemon = 1,
 		/mob/living/simple_animal/hostile/asteroid/hivelord/legion = 1,
 	)