utility/file_manager library

1. mix_player package
2. documentation
3. utility/file_manager.dart

mix_player package

1. Libraries
2. mix_player
3. mix_service
4. models/audio_item
5. models/download_task
6. models/extension
7. models/filemanager_task
8. models/mix_item
9. models/PlaybackEventMessage
10. models/player_state
11. models/request_song
12. notification/NotificationService
13. player_audio
14. utility/file_manager