dehardcodes limb icons (tgstation#61570)

This is a small change aimed at helping downstream modularity.
Currently the file these sprites use are hardcoded with a define, this removes that and instead moves it to a var.



I guess the thing is defaults should be overridable, it's just we've never ran into a case where it's needed, so adding that functionality on its own looks really weird

annoying. I don't like adding stuff that's only useful to downstreams, for various reasons, but this seems ok after some thought. fulfills the promise of _DEFAULT.
-Lemon

code/__DEFINES/mobs.dm

@@ -59,7 +59,6 @@
 #define DEFAULT_BODYPART_ICON_ORGANIC 'icons/mob/human_parts_greyscale.dmi'
 #define DEFAULT_BODYPART_ICON_ROBOTIC 'icons/mob/augmentation/augments.dmi'
 
-
 #define MONKEY_BODYPART "monkey"
 #define ALIEN_BODYPART "alien"
 #define LARVA_BODYPART "larva"

code/modules/surgery/bodyparts/_bodyparts.dm

@@ -1,4 +1,3 @@
-
 /obj/item/bodypart
 	name = "limb"
 	desc = "Why is it detached..."
@@ -7,6 +6,10 @@
 	w_class = WEIGHT_CLASS_SMALL
 	icon = 'icons/mob/human_parts.dmi'
 	icon_state = ""
+	/// The icon for Organic limbs using greyscale
+	var/icon_greyscale = DEFAULT_BODYPART_ICON_ORGANIC
+	/// The icon for Robotic limbs
+	var/icon_robotic = DEFAULT_BODYPART_ICON_ROBOTIC
 	layer = BELOW_MOB_LAYER //so it isn't hidden behind objects when on the floor
 	grind_results = list(/datum/reagent/bone_dust = 10, /datum/reagent/liquidgibs = 5) // robotic bodyparts and chests/heads cannot be ground
 	var/mob/living/carbon/owner
@@ -713,9 +716,9 @@
 
 	if(change_icon_to_default)
 		if(status == BODYPART_ORGANIC)
-			icon = DEFAULT_BODYPART_ICON_ORGANIC
+			icon = icon_greyscale
 		else if(status == BODYPART_ROBOTIC)
-			icon = DEFAULT_BODYPART_ICON_ROBOTIC
+			icon = icon_robotic
 
 	if(owner)
 		owner.updatehealth()
@@ -880,7 +883,7 @@
 		return
 
 	if(should_draw_greyscale)
-		limb.icon = 'icons/mob/human_parts_greyscale.dmi'
+		limb.icon = icon_greyscale
 		if(should_draw_gender)
 			limb.icon_state = "[species_id]_[body_zone]_[icon_gender]"
 		else if(use_digitigrade)