Remove IStartCollide from flammable (#4314)

I ported some stuff to be ECS but didn't feel like porting the rest, sue me.

Content.Server/Atmos/EntitySystems/FlammableSystem.cs

@@ -0,0 +1,56 @@
+using Content.Server.Atmos.Components;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Physics.Dynamics;
+
+namespace Content.Server.Atmos.EntitySystems
+{
+    internal sealed class FlammableSystem : EntitySystem
+    {
+        // TODO: Port the rest of Flammable.
+        public override void Initialize()
+        {
+            base.Initialize();
+            SubscribeLocalEvent<FlammableComponent, StartCollideEvent>(HandleCollide);
+        }
+
+        private void HandleCollide(EntityUid uid, FlammableComponent component, StartCollideEvent args)
+        {
+            if (!args.OtherFixture.Body.Owner.TryGetComponent(out FlammableComponent? otherFlammable))
+                return;
+
+            if (!component.FireSpread || !otherFlammable.FireSpread)
+                return;
+
+            if (component.OnFire)
+            {
+                if (otherFlammable.OnFire)
+                {
+                    var fireSplit = (component.FireStacks + otherFlammable.FireStacks) / 2;
+                    component.FireStacks = fireSplit;
+                    otherFlammable.FireStacks = fireSplit;
+                }
+                else
+                {
+                    component.FireStacks /= 2;
+                    otherFlammable.FireStacks += component.FireStacks;
+                    Ignite(otherFlammable);
+                }
+            } else if (otherFlammable.OnFire)
+            {
+                otherFlammable.FireStacks /= 2;
+                component.FireStacks += otherFlammable.FireStacks;
+                Ignite(component);
+            }
+        }
+
+        internal void Ignite(FlammableComponent component)
+        {
+            if (component.FireStacks > 0 && !component.OnFire)
+            {
+                component.OnFire = true;
+            }
+
+            component.UpdateAppearance();
+        }
+    }
+}