Make `DigestSink` implement `Sink` (#146)
In Dart 3, `Sink` is an interface class, and cannot be extended.
diff --git a/lib/src/digest_sink.dart b/lib/src/digest_sink.dart
index 9ba4df9..092fc40 100644
--- a/lib/src/digest_sink.dart
+++ b/lib/src/digest_sink.dart
@@ -5,7 +5,7 @@
import 'digest.dart';
/// A sink used to get a digest value out of `Hash.startChunkedConversion`.
-class DigestSink extends Sink<Digest> {
+class DigestSink implements Sink<Digest> {
/// The value added to the sink.
///
/// A value must have been added using [add] before reading the `value`.
