fix: Changes Map.Sector.Items to link list. (#1547)

### Summary

Eliminates `IPooledEnumerable<T>` and `eable.Free()` from `Map` for items. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var item in m.GetItemsInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.

Projects/Server/Geometry/Rectangle2D.cs

@@ -22,6 +22,8 @@ namespace Server;
 [PropertyObject]
 public struct Rectangle2D : IEquatable<Rectangle2D>, ISpanFormattable, ISpanParsable<Rectangle2D>
 {
+    public static Rectangle2D Empty => new();
+
     private Point2D _start;
     private Point2D _end;