Add nullable context to SabreTools.DatItems

This change also starts migrating the internals of the DatItem formats to the new internal models. Right now, it's basically just acting like a wrapper around those models.

SabreTools.Models/Internal/RamOption.cs

@@ -8,6 +8,9 @@ namespace SabreTools.Models.Internal
     {
         #region Keys
 
+        /// <remarks>string</remarks>
+        public const string ContentKey = "content";
+
         /// <remarks>(yes|no) "no"</remarks>
         public const string DefaultKey = "default";