Add fetchSublist method to BinaryTable for improved data retrieval

dart/lib/binary_table.dart

@@ -210,6 +210,17 @@ class BT_Reference {
     }
   }
 
+  BT_Type? get type {
+    if (_pointer.isNull) {
+      return null;
+    }
+
+    _table._file.setPositionSync(_pointer.address);
+    int typeId = _table._file.readByteSync();
+    return BT_Type.fromId(typeId);
+  }
+
+
   @override
   String toString() => _pointer.toString();
 }