Support AVAudioPlayer.pan

[actow]

Is your feature request related to a problem? Please describe.
lack of the ability to adjust volume between the left and right channels

Describe the solution you'd like
Expose AVAudioPlayer.pan
https://developer.apple.com/documentation/avfaudio/avaudioplayer/1390884-pan

Describe alternatives you've considered
none

Additional context
This would enable the creation of basic 3D sound effects.

[actow]

On Android. It's probably MediaPlayer.setVolume(left, right) or SoundPool.play(...)

[ryanheise]

I have implemented this on Android although it is not yet pushed to GitHub. For iOS I do not use AVAudioPlayer so this solution won't work. However, this will be easier once #334 is done.

Note: I am currently handling this issue in #126 so please add your use case to that issue.

[github-actions]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs, or use StackOverflow if you need help with just_audio.