feat: add MultiPlateResult type for part-first nesting

OpenNest.Engine/MultiPlateResult.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+
+namespace OpenNest
+{
+    public class MultiPlateResult
+    {
+        public List<PlateResult> Plates { get; set; } = new();
+        public List<NestItem> UnplacedItems { get; set; } = new();
+    }
+
+    public class PlateResult
+    {
+        public Plate Plate { get; set; }
+        public List<Part> Parts { get; set; } = new();
+        public PlateOption ChosenSize { get; set; }
+        public bool IsNew { get; set; }
+    }
+}