Dropship Missile Refactor (#3732)

# About the pull request
Refactors Dropship Missiles to not use Spawn().
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# Explain why it's good for the game
While proposing a new CAS feature, this part of the code uses a function
we shouldn't be using. With a suggestion from Harry I simply swapped out
the parts that we could replace without having to do a bigger overhaul.
# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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# Changelog
:cl:
refactor: refactored dropship_ammo so that missiles don't use Spawn()
/:cl:

code/modules/cm_marines/dropship_ammo.dm

@@ -287,9 +287,8 @@
 
 /obj/structure/ship_ammo/rocket/widowmaker/detonate_on(turf/impact)
 	impact.ceiling_debris_check(3)
-	spawn(5)
-		cell_explosion(impact, 300, 40, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name), source_mob)) //Your standard HE splash damage rocket. Good damage, good range, good speed, it's an all rounder
-		qdel(src)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(cell_explosion), impact, 300, 40, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name)), source_mob), 0.5 SECONDS) //Your standard HE splash damage rocket. Good damage, good range, good speed, it's an all rounder
+	QDEL_IN(src, 0.5 SECONDS)
 
 /obj/structure/ship_ammo/rocket/banshee
 	name = "\improper AGM-227 'Banshee'"
@@ -301,10 +300,9 @@
 
 /obj/structure/ship_ammo/rocket/banshee/detonate_on(turf/impact)
 	impact.ceiling_debris_check(3)
-	spawn(5)
-		cell_explosion(impact, 175, 20, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name), source_mob)) //Small explosive power with a small fall off for a big explosion range
-		fire_spread(impact, create_cause_data(initial(name), source_mob), 4, 15, 50, "#00b8ff") //Very intense but the fire doesn't last very long
-		qdel(src)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(cell_explosion), impact, 175, 20, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name)), source_mob), 0.5 SECONDS) //Small explosive power with a small fall off for a big explosion range
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(fire_spread), impact, create_cause_data(initial(name), source_mob), 4, 15, 50, "#00b8ff"), 0.5 SECONDS) //Very intense but the fire doesn't last very long
+	QDEL_IN(src, 0.5 SECONDS)
 
 /obj/structure/ship_ammo/rocket/keeper
 	name = "\improper GBU-67 'Keeper II'"
@@ -317,9 +315,8 @@
 
 /obj/structure/ship_ammo/rocket/keeper/detonate_on(turf/impact)
 	impact.ceiling_debris_check(3)
-	spawn(5)
-		cell_explosion(impact, 450, 100, EXPLOSION_FALLOFF_SHAPE_EXPONENTIAL, null, create_cause_data(initial(name), source_mob)) //Insane fall off combined with insane damage makes the Keeper useful for single targets, but very bad against multiple.
-		qdel(src)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(cell_explosion), impact, 450, 100, EXPLOSION_FALLOFF_SHAPE_EXPONENTIAL, null, create_cause_data(initial(name)), source_mob), 0.5 SECONDS) //Insane fall off combined with insane damage makes the Keeper useful for single targets, but very bad against multiple.
+	QDEL_IN(src, 0.5 SECONDS)
 
 /obj/structure/ship_ammo/rocket/harpoon
 	name = "\improper AGM-84 'Harpoon'"
@@ -333,9 +330,8 @@
 //Looks kinda OP but all it can actually do is just to blow windows and some of other things out, cant do much damage.
 /obj/structure/ship_ammo/rocket/harpoon/detonate_on(turf/impact)
 	impact.ceiling_debris_check(3)
-	spawn(5)
-		cell_explosion(impact, 150, 16, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name), source_mob))
-		qdel(src)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(cell_explosion), impact, 150, 16, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name)), source_mob), 0.5 SECONDS)
+	QDEL_IN(src, 0.5 SECONDS)
 
 /obj/structure/ship_ammo/rocket/napalm
 	name = "\improper XN-99 'Napalm'"
@@ -347,10 +343,9 @@
 
 /obj/structure/ship_ammo/rocket/napalm/detonate_on(turf/impact)
 	impact.ceiling_debris_check(3)
-	spawn(5)
-		cell_explosion(impact, 200, 25, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name), source_mob))
-		fire_spread(impact, create_cause_data(initial(name), source_mob), 6, 60, 30, "#EE6515") //Color changed into napalm's color to better convey how intense the fire actually is.
-		qdel(src)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(cell_explosion), impact, 200, 25, EXPLOSION_FALLOFF_SHAPE_LINEAR, null, create_cause_data(initial(name)), source_mob), 0.5 SECONDS)
+	addtimer(CALLBACK(GLOBAL_PROC, GLOBAL_PROC_REF(fire_spread), impact, create_cause_data(initial(name), source_mob), 6, 60, 30, "#EE6515"), 0.5 SECONDS) //Color changed into napalm's color to better convey how intense the fire actually is.
+	QDEL_IN(src, 0.5 SECONDS)