fix: Memory leak on darwin (#1969)

# Description

We at Elredo sometime run into weird issues regarding audio playing
using this library. So I started to investigate and I found out that
there might be few memory leaks in the library.

Author: kubito

packages/audioplayers_darwin/darwin/audioplayers_darwin/Sources/audioplayers_darwin/AudioplayersDarwinPlugin.swift

@@ -73,7 +73,10 @@ public class AudioplayersDarwinPlugin: NSObject, FlutterPlugin {
   }
 
   public func detachFromEngine(for registrar: FlutterPluginRegistrar) {
-    Task { @MainActor in
+    Task { @MainActor [weak self] in
+      guard let self = self else {
+        return
+      }
       await disposePlayers()
       self.globalMethods.setMethodCallHandler(nil)
       self.globalEvents.dispose()

packages/audioplayers_darwin/darwin/audioplayers_darwin/Sources/audioplayers_darwin/WrappedMediaPlayer.swift

@@ -197,11 +197,14 @@ enum ReleaseMode: String {
   ) async throws {
     try await withCheckedThrowingContinuation { continuation in
       playerItemStatusObservation = playerItem.observe(\AVPlayerItem.status) {
-        (playerItem, change) in
+        [weak self] (playerItem, change) in
+        guard let self = self else {
+          return
+        }
         let status = playerItem.status
         self.eventHandler.onLog(message: "player status: \(status), change: \(change)")
 
-        switch playerItem.status {
+        switch status {
         case .readyToPlay:
           continuation.resume()
         case .failed: