Port one more extension from UnshieldSharp

SabreTools.Serialization/Wrappers/InstallShieldCabinet.cs

@@ -41,6 +41,11 @@ namespace SabreTools.Serialization.Wrappers
         /// </summary>
         public int FileGroupCount => Model.FileGroups?.Length ?? 0;
 
+        /// <summary>
+        /// Indicates if Unicode strings are used
+        /// </summary>
+        public bool IsUnicode => MajorVersion >= 17;
+
         /// <summary>
         /// The major version of the cabinet
         /// </summary>