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/DictionaryBase.cs

@@ -93,6 +93,9 @@ namespace SabreTools.Models.Internal
             if (asArray != null)
                 return string.Join(',', asArray);
 
+            // TODO: Add byte array conversion here
+            // TODO: Add byte array read helper
+
             return this[key]!.ToString();
         }