Add NearestChunkEnumerator (#5972)

Not super fast but want it for biome loading to prio chunks nearby first.

Robust.Shared/Map/Enumerators/NearestChunkEnumerator.cs

@@ -0,0 +1,64 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Numerics;
+using Robust.Shared.Collections;
+using Robust.Shared.Maths;
+
+namespace Robust.Shared.Map.Enumerators;
+
+/// <summary>
+/// Iterates chunk indices but prefers ones closer towards the center first.
+/// </summary>
+public record struct NearestChunkEnumerator
+{
+    private readonly Vector2i _chunkLB;
+    private readonly Vector2i _chunkRT;
+
+    private ValueList<Vector2i> _chunks = new();
+
+    private int _n;
+
+    public NearestChunkEnumerator(Box2 localAABB, int chunkSize)
+    {
+        _chunkLB = (localAABB.BottomLeft / chunkSize).Floored();
+        _chunkRT = (localAABB.TopRight / chunkSize).Floored();
+        InitializeChunks(new Vector2i(chunkSize, chunkSize));
+    }
+
+    public NearestChunkEnumerator(Box2 localAABB, Vector2i chunkSize)
+    {
+        _chunkLB = (localAABB.BottomLeft / chunkSize).Floored();
+        _chunkRT = (localAABB.TopRight / chunkSize).Floored();
+        InitializeChunks(chunkSize);
+    }
+
+    private void InitializeChunks(Vector2i chunkSize)
+    {
+        var bl = (Vector2)_chunkLB * chunkSize;
+        var tr = (Vector2)_chunkRT * chunkSize;
+        var halfChunk = new Vector2(chunkSize.X / 2f, chunkSize.Y / 2f);
+
+        var center = (tr - bl) / 2 + bl;
+
+        for (var x = _chunkLB.X; x < _chunkRT.X; x++)
+        {
+            for (var y = _chunkLB.Y; y < _chunkRT.Y; y++)
+            {
+                _chunks.Add(new Vector2i(x, y) * chunkSize);
+            }
+        }
+
+        _chunks.Sort((c1, c2) => ((c1 + halfChunk) - center).LengthSquared().CompareTo(((c2 + halfChunk) - center).LengthSquared()));
+    }
+
+    public bool MoveNext([NotNullWhen(true)] out Vector2i? indices)
+    {
+        if (_n >= _chunks.Count)
+        {
+            indices = null;
+            return false;
+        }
+
+        indices = _chunks[_n++] ;
+        return true;
+    }
+}