Death of mutant bodyparts AND external organs (#85137)

## About The Pull Request

Removes mutant bodyparts and external organs from the game completely
Digitgrade behaviour was mutant bodypart for no reason

Cat ears now work with the bodyparts overlay system, same as all the
other external organs (since all their behaviour is now just on /organ

It doesn't remove all the /external types, but moves all behaviour to
/organ. I'll follow up with a PR wiping all the /external organ types,
but it's just conflict heaven so not this PR

I've also streamlined a lot of duplicate/weird species regeneration code

Melbert did the same PR as well but due to a lack of time (?) I have
absorbed his PR to double nuke mutant bodyparts

## Why It's Good For The Game
Frees us from the chain of unmodular code, and kills my greatest nemesis
(after the shuttle meteor murder bug)

## Changelog
🆑 Time-Green and MrMelbert
Refactor: External organ behaviour has been moved to /organ, ears now
use the same system as the other organs
Refactor: Mutant bodyparts are dead! This likely does not mean much to
the average person but it's very dear to me
code: Improves digitgrade handling in preference code
/🆑

I have absorbed #85126, using Melberts code to improve and add some
missing changes. Mainly improving the functioning of preferences and
digitgrade legs. I didn't take over the hairstyle improvements.

---------

Co-authored-by: Ghom <42542238+Ghommie@users.noreply.github.com>

code/modules/unit_tests/organ_bodypart_shuffle.dm

@@ -2,7 +2,7 @@
 /datum/unit_test/organ_bodypart_shuffle
 
 /datum/unit_test/organ_bodypart_shuffle/Run()
-	var/mob/living/carbon/human/hollow_boy = allocate(/mob/living/carbon/human/consistent)
+	var/mob/living/carbon/human/hollow_boy = allocate(/mob/living/carbon/human/consistent) //freshly filled with wet insides
 
 	// Test if organs are all properly updating when forcefully removed
 	var/list/removed_organs = list()
@@ -30,5 +30,3 @@
 				continue
 			TEST_ASSERT(organ in hollow_boy.organs, "Organ '[organ.name] was put in an empty bodypart that replaced a humans, but the organ did not come with.")
 
-	// Test if bodyparts are all properly updating when forcefully removed
-	hollow_boy = allocate(/mob/living/carbon/human/consistent) //freshly filled with wet insides

code/modules/unit_tests/organ_set_bonus.dm

@@ -30,7 +30,7 @@
 		// Attempt to insert entire list of mutant organs for the given infusion_entry.
 		for(var/obj/item/organ/organ as anything in output_organs)
 			organ = new organ()
-			TEST_ASSERT(organ.Insert(lab_rat, special = TRUE, movement_flags = DELETE_IF_REPLACED), "The organ `[organ.type]` for `[infuser_entry.type]` was not inserted in the mob when expected, Insert() returned falsy when TRUE was expected.")
+			organ.Insert(lab_rat, special = TRUE, movement_flags = DELETE_IF_REPLACED)
 			inserted_organs += organ
 
 		// Search for added Status Effect.