Add managed entity and script component.

Added ManagedEntity and related methods in EntityManager;
Added ScriptComponent to write game play logic in oop;

Ghost.Entities/ManagedEntity.cs

@@ -0,0 +1,74 @@
+using System.Runtime.CompilerServices;
+
+namespace Ghost.Entities;
+
+public record struct ManagedEntity
+{
+    public int id;
+    public int generation;
+
+    public override readonly string ToString()
+    {
+        return $"ManagedEntity({id}, {generation})";
+    }
+}
+
+public struct ManagedEntityRef : IComponent
+{
+    public ManagedEntity entity;
+}
+
+public abstract class ScriptComponent : IComponent
+{
+    internal World _world = null!;
+    internal Entity _entity;
+    internal ManagedEntity _managedEntity;
+
+    public World World => _world;
+    public Entity Entity => _entity;
+    public ManagedEntity ManagedEntity => _managedEntity;
+
+    protected ref T GetComponent<T>()
+        where T : unmanaged, IComponent
+    {
+        ref var value = ref _world.EntityManager.GetComponent<T>(_entity);
+        if (Unsafe.IsNullRef(ref value))
+        {
+            throw new InvalidOperationException($"Entity {_entity} does not have component of type {typeof(T)}");
+        }
+
+        return ref value;
+    }
+
+    public virtual void OnCreate()
+    {
+    }
+
+    public virtual void OnDestroy()
+    {
+    }
+
+    public virtual void OnEnable()
+    {
+    }
+
+    public virtual void OnDisable()
+    {
+    }
+
+    public virtual void Start()
+    {
+    }
+
+    public virtual void Update()
+    {
+    }
+
+    public virtual void FixedUpdate()
+    {
+    }
+
+    public virtual void LateUpdate()
+    {
+    }
+}