refactor: behaviour collector Count and indexer accessors added

Engine.Core/Abstract/IBehaviourCollector.cs

@@ -1,5 +1,3 @@
-using System.Collections.Generic;
-
 namespace Syntriax.Engine.Core;
 
 /// <summary>
@@ -19,6 +17,16 @@ public interface IBehaviourCollector<T> : IHasUniverse where T : class
     /// </summary>
     event RemovedEventHandler? OnRemoved;
 
+    /// <summary>
+    /// Amount of <typeparamref name="T"/> collected.
+    /// </summary>
+    int Count { get; }
+
+    /// <summary>
+    /// Get a <typeparamref name="T"/> collected by it's index.
+    /// </summary>
+    T this[System.Index index] { get; }
+
     /// <summary>
     /// Delegate for handling the <see cref="OnCollected"/> event.
     /// </summary>