chore: Added Initial Engine Code

Engine.Core/Abstract/IBehaviour.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace Syntriax.Engine.Core.Abstract;
+
+/// <summary>
+/// Responsible for every behaviour an object in the game might have, controlled by <see cref="IBehaviourController"/>.
+/// </summary>
+public interface IBehaviour : IEntity, IAssignableBehaviourController, IAssignableStateEnable, IInitialize
+{
+    /// <summary>
+    /// Callback triggered when the <see cref="Priority"/> has changed.
+    /// </summary>
+    Action<IBehaviour>? OnPriorityChanged { get; set; }
+
+    /// <summary>
+    /// Call priority of the <see cref="IBehaviour"/>.
+    /// </summary>
+    int Priority { get; set; }
+}