Start creating Dictionary-based internal models

SabreTools.Models/Internal/DiskArea.cs

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+using System.Xml.Serialization;
+using Newtonsoft.Json;
+
+namespace SabreTools.Models.Internal
+{
+    [JsonObject("diskarea"), XmlRoot("diskarea")]
+    public class DiskArea : DatItem
+    {
+        #region Keys
+
+        /// <remarks>Disk[]</remarks>
+        public const string DiskKey = "disk";
+
+        /// <remarks>string</remarks>
+        public const string NameKey = "name";
+
+        #endregion
+
+        public DiskArea() => Type = ItemType.DiskArea;
+    }
+}