Cuts the number of apply_damage copypaste procs from 3(.5) to 1, fixing a few bugs along the way (#79207)

## About The Pull Request

- `apply_damage` is now uniform, handled, in a single place. At the
living level.
- Fixes people being held at gunpoint from being unable to flinch from
taking damage.
- Fixes a few signals (explodable, glass jaw) of apply damage maybe
potentially not having effects if the passed hit zone was a bodypart and
not a def zone.
- Fixes regenerator component always halting regeneration no matter what
damage taken.
- Fixes pressure damage being unaffected by `brute_mod` despite what
documentation claims.
- Signalizes the `check_species_weakness` proc on species. 

## Why It's Good For The Game

The copy+paste of this proc made it awful, awful to maintain or add
features to. And also made it very bug prone.

This just takes a step forward in making sustaining damage a lot more
consistent, not only across all mobs, but across all methods of...
taking damage. Unarmed attacks, hulk, item attacks, etc.

## Changelog

🆑 Melbert
fix: People held at gunpoint can now flinch when being hit. 
fix: Regenerating mobs no longer stop regenerating no matter hit with
what.
fix: Pressure damage is now properly modified by a mob's brute damage
modifier.
fix: Fixes some occasions which some effects (glass jaw, explodable worn
items) won't respond to hits.
refactor: Refactored core code related to mobs sustaining damage. 
/🆑

code/datums/components/explodable.dm

@@ -91,13 +91,16 @@
 		detonate()
 
 ///Called when you attack a specific body part of the thing this is equipped on. Useful for exploding pants.
-/datum/component/explodable/proc/explodable_attack_zone(datum/source, damage, damagetype, def_zone)
+/datum/component/explodable/proc/explodable_attack_zone(datum/source, damage, damagetype, def_zone, ...)
 	SIGNAL_HANDLER
 
 	if(!def_zone)
 		return
 	if(damagetype != BURN) //Don't bother if it's not fire.
 		return
+	if(isbodypart(def_zone))
+		var/obj/item/bodypart/hitting = def_zone
+		def_zone = hitting.body_zone
 	if(!is_hitting_zone(def_zone)) //You didn't hit us! ha!
 		return
 	detonate()