fix(web): make start and resume async (#1436)

If playing an invalid file (e.g. an empty file), the error is not
propagated back to the library, as the error is thrown when starting the
`play` / `resume` operation.
This ensures that the future of web js `play` is awaited.

packages/audioplayers_web/lib/audioplayers_web.dart

@@ -53,7 +53,7 @@ class AudioplayersPlugin extends AudioplayersPlatform with StreamsInterface {
 
   @override
   Future<void> resume(String playerId) async {
-    getOrCreatePlayer(playerId).resume();
+    await getOrCreatePlayer(playerId).resume();
   }
 
   @override

packages/audioplayers_web/lib/wrapped_player.dart

@@ -127,20 +127,20 @@ class WrappedPlayer {
     playerPlaySubscription = null;
   }
 
-  void start(double position) {
+  Future<void> start(double position) async {
     isPlaying = true;
     if (currentUrl == null) {
       return; // nothing to play yet
     }
     if (player == null) {
       recreateNode();
     }
-    player?.play();
+    await player?.play();
     player?.currentTime = position;
   }
 
-  void resume() {
-    start(pausedAt ?? 0);
+  Future<void> resume() async {
+    await start(pausedAt ?? 0);
   }
 
   void pause() {