Fix Fultons losing their beacon when split (#20179)

Content.Shared/Stacks/StackSplitEvent.cs

@@ -0,0 +1,8 @@
+namespace Content.Shared.Stacks;
+
+/// <summary>
+///     Raised on the original stack entity when it is split to create another.
+/// </summary>
+/// <param name="NewId">The entity id of the new stack.</param>
+[ByRefEvent]
+public readonly record struct StackSplitEvent(EntityUid NewId);