ECS StunnableProjectile (#4309)

* ECS StunnableProjectile

Also renamed it to the more generic component for stunning on collision

* Reviews

Content.Server/Stunnable/StunOnCollideSystem.cs

@@ -0,0 +1,27 @@
+using Content.Server.Stunnable.Components;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Physics.Dynamics;
+
+namespace Content.Server.Stunnable
+{
+    [UsedImplicitly]
+    internal sealed class StunOnCollideSystem : EntitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+            SubscribeLocalEvent<StunOnCollideComponent, StartCollideEvent>(HandleCollide);
+        }
+
+        private void HandleCollide(EntityUid uid, StunOnCollideComponent component, StartCollideEvent args)
+        {
+            if (args.OtherFixture.Body.Owner.TryGetComponent(out StunnableComponent? stunnableComponent))
+            {
+                stunnableComponent.Stun(component.StunAmount);
+                stunnableComponent.Knockdown(component.KnockdownAmount);
+                stunnableComponent.Slowdown(component.SlowdownAmount);
+            }
+        }
+    }
+}