AutocompleteCore (#62927)

A new widget that chooses an item from a list based on text input. Just the core widget, with Material and Cupertino versions to come.

packages/flutter/lib/widgets.dart

@@ -23,6 +23,7 @@ export 'src/widgets/animated_switcher.dart';
 export 'src/widgets/annotated_region.dart';
 export 'src/widgets/app.dart';
 export 'src/widgets/async.dart';
+export 'src/widgets/autocomplete.dart';
 export 'src/widgets/autofill.dart';
 export 'src/widgets/automatic_keep_alive.dart';
 export 'src/widgets/banner.dart';