feat: added a wip nested coroutine manager

This coroutine manager allows for nested IEnumerators, however it is experimental at the moment.

Engine.Core/Systems/NestedCoroutineManager.cs

@@ -0,0 +1,80 @@
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Engine.Core;
+
+public class NestedCoroutineManager : Behaviour, IUpdate, ICoroutineManager
+{
+    private readonly List<CoroutineStack> stacks = [];
+    private readonly Pool<CoroutineStack> pool = new(() => new());
+
+    public IEnumerator StartCoroutine(IEnumerator enumerator)
+    {
+        CoroutineStack stack = pool.Get();
+        stack.EntryPoint = enumerator;
+        stack.Stack.Push(enumerator);
+
+        stacks.Add(stack);
+
+        return enumerator;
+    }
+
+    public void StopCoroutine(IEnumerator enumerator)
+    {
+        for (int i = 0; i < stacks.Count; i++)
+            if (stacks[i].EntryPoint == enumerator)
+            {
+                RemoveCoroutineAt(i);
+                return;
+            }
+    }
+
+    private void RemoveCoroutineAt(int i)
+    {
+        stacks[i].Reset();
+        stacks.RemoveAt(i);
+    }
+
+    void IUpdate.Update()
+    {
+        for (int i = stacks.Count - 1; i >= 0; i--)
+        {
+            Stack<IEnumerator> stack = stacks[i].Stack;
+
+            if (stack.Count == 0)
+            {
+                RemoveCoroutineAt(i);
+                continue;
+            }
+
+            IEnumerator top = stack.Peek();
+
+            if (top.Current is ICoroutineYield coroutineYield && coroutineYield.Yield())
+                continue;
+
+            if (top.Current is IEnumerator nested)
+            {
+                stack.Push(nested);
+                continue;
+            }
+
+            if (!top.MoveNext())
+            {
+                stack.Pop();
+                if (stack.Count != 0)
+                    stack.Peek().MoveNext();
+                continue;
+            }
+        }
+    }
+
+    private class CoroutineStack
+    {
+        public IEnumerator EntryPoint = null!;
+        public Stack<IEnumerator> Stack = new();
+
+        public void Reset() { EntryPoint = null!; Stack.Clear(); }
+    }
+
+    public NestedCoroutineManager() => Priority = int.MinValue;
+}