Explicitly export Kind; for internal customer (#3431)

Author: srawlins

lib/dartdoc.dart

@@ -12,6 +12,7 @@ export 'package:dartdoc/src/dartdoc_options.dart';
     'the public API starting in dartdoc 7.0.0')
 export 'package:dartdoc/src/element_type.dart';
 export 'package:dartdoc/src/generator/generator.dart';
+export 'package:dartdoc/src/model/indexable.dart' show Kind;
 @Deprecated(
     'Elements which are only exposed via this export will no longer be part of '
     'the public API starting in dartdoc 7.0.0')