Christmas event changes (#33861)

* A christmas tree with presents is now a subtype

Admins can now spawn `/obj/structure/flora/tree/pine/xmas/presents`
directly, instead of spawning regular christmas trees and then
triggering the event.

Also, the event now just deletes and replaces the xmas tree with a
present tree; saves on duplication of icon states and vars and all that.

* Code review I

* Gaia's Landing: The PR

code/modules/holiday/holidays.dm

@@ -403,6 +403,17 @@ Since Ramadan is an entire month that lasts 29.5 days on average, the start and
 /datum/holiday/xmas/greet()
 	return "Have a merry Christmas!"
 
+/datum/holiday/xmas/celebrate()
+	SSticker.OnRoundstart(CALLBACK(src, .proc/roundstart_celebrate))
+
+/datum/holiday/xmas/proc/roundstart_celebrate()
+	for(var/obj/machinery/computer/security/telescreen/entertainment/Monitor in GLOB.machines)
+		Monitor.icon_state = "entertainment_xmas"
+
+	for(var/mob/living/simple_animal/pet/dog/corgi/Ian/Ian in GLOB.mob_living_list)
+		Ian.place_on_head(new /obj/item/clothing/head/helmet/space/santahat(Ian))
+
+
 /datum/holiday/festive_season
 	name = FESTIVE_SEASON
 	begin_day = 1