chore: improve testability by exposing SingleData and ManyData on Document

Author: maurei

src/JsonApiDotNetCore/Models/JsonApiDocuments/ExposableData.cs

@@ -31,12 +31,12 @@ public bool ShouldSerializeData()
         /// <summary>
         /// Internally used for "single" primary data.
         /// </summary>
-        internal T SingleData { get; private set; }
+        public T SingleData { get; private set; }
 
         /// <summary>
         /// Internally used for "many" primary data.
         /// </summary>
-        internal List<T> ManyData { get; private set; }
+        public List<T> ManyData { get; private set; }
 
         /// <summary>
         /// Internally used to indicate if the document's primary data is