Adds Source Count Macro (#77519)

## About The Pull Request

I prefer this to having people manually read the list, plus it seems
like reasonable information to expose, especially if we're gonna start
using it in this pattern.

## Why It's Good For The Game

Less futzing with internals

code/__DEFINES/traits.dm

@@ -115,7 +115,9 @@
 #define HAS_TRAIT_NOT_FROM(target, trait, source) (HAS_TRAIT(target, trait) && (length(target._status_traits[trait] - source) > 0))
 /// Returns a list of trait sources for this trait. Only useful for wacko cases and internal futzing
 /// You should not be using this
-#define GET_TRAIT_SOURCES(target, trait) target._status_traits?[trait] || list()
+#define GET_TRAIT_SOURCES(target, trait) (target._status_traits?[trait] || list())
+/// Returns the amount of sources for a trait. useful if you don't want to have a "thing counter" stuck around all the time
+#define COUNT_TRAIT_SOURCES(target, trait) length(GET_TRAIT_SOURCES(target, trait))
 /// A simple helper for checking traits in a mob's mind
 #define HAS_MIND_TRAIT(target, trait) (HAS_TRAIT(target, trait) || (target.mind ? HAS_TRAIT(target.mind, trait) : FALSE))
 

code/datums/elements/sticker.dm

@@ -39,7 +39,7 @@
 
 ///Add our stick_type to the target with px and py as pixel x and pixel y respectively
 /datum/element/sticker/proc/do_stick(obj/item/source, atom/target, mob/living/user, px, py)
-	if(length(GET_TRAIT_SOURCES(target, TRAIT_STICKERED)) >= MAX_ALLOWED_STICKERS)
+	if(COUNT_TRAIT_SOURCES(target, TRAIT_STICKERED) >= MAX_ALLOWED_STICKERS)
 		source.balloon_alert_to_viewers("sticker won't stick!")
 		return FALSE
 	target.AddComponent(stick_type, px, py, source, user, washable)