Add support for multiple explosions to CreateEffectWarhead

Have the engine pick one explosion at random from the provided list.

mods/ts/weapons/superweapons.yaml

@@ -25,11 +25,11 @@ MultiCluster:
 			Concrete: 60
 		DamageTypes: Prone50Percent, TriggerProne, ExplosionDeath
 	Warhead@2Eff: CreateEffect
-		Explosion: large_explosion
+		Explosions: large_explosion
 		ImpactSound: expnew09.aud
 		InvalidImpactTypes: Water
 	Warhead@3EffWater: CreateEffect
-		Explosion: small_watersplash
+		Explosions: small_watersplash
 		ExplosionPalette: player
 		ImpactSound: ssplash2.aud
 		ValidImpactTypes: Water
@@ -61,7 +61,7 @@ IonCannon:
 		ValidTargets: Ground, Air
 		DamageTypes: Prone100Percent, TriggerProne, EnergyDeath
 	Warhead@2Eff_impact: CreateEffect
-		Explosion: ionring
+		Explosions: ionring
 		ImpactSound: ion1.aud
 	Warhead@3Dam_area: SpreadDamage
 		Spread: 1c0
@@ -86,7 +86,7 @@ EMPulseCannon:
 		Angle: 62
 		Image: pulsball
 	Warhead@1Eff: CreateEffect
-		Explosion: pulse_explosion
+		Explosions: pulse_explosion
 	Warhead@emp: GrantUpgrade
 		Range: 3c0
 		Duration: 250
@@ -103,7 +103,7 @@ ClusterMissile:
 			Concrete: 25
 		DamageTypes: Prone50Percent, TriggerProne, FireDeath
 	Warhead@SoundEffect0: CreateEffect
-		Explosion: large_explosion
+		Explosions: large_explosion
 		ImpactSound: expnew19.aud
 	Warhead@ResourceDestruction0: DestroyResource
 		Size: 1