player: Add stop method.

musicus_player/android/src/main/kotlin/de/johrpan/musicus_player/MusicusPlayerPlugin.kt

@@ -47,6 +47,11 @@ public class MusicusPlayerPlugin: FlutterPlugin, MethodCallHandler {
     } else if (call.method == "pause") {
       mediaPlayer?.pause()
       result.success(null)
+    } else if (call.method == "stop") {
+      uri = null
+      mediaPlayer?.release()
+      mediaPlayer = null
+      result.success(null)
     } else {
       result.notImplemented()
     }

musicus_player/lib/musicus_player.dart

@@ -40,4 +40,9 @@ class MusicusPlayer {
   Future<void> pause() async {
     await _channel.invokeMethod('pause');
   }
+
+  /// Stop the player.
+  Future<void> stop() async {
+    await _channel.invokeMethod('stop');
+  }
 }