refactor!: improve separation of global audioplayer interface

[Gustl22]

Description

AudioPlayers platform communication consists of two parts: the AudioPlayer scope and the Global scope.
These were not separated consistently, so one was used as implementation and the other as interface.
Now both scopes are handled the same way:

• AudioPlayer: player specific processing and interaction via AudioplayersPlatformInterface
• GlobalAudioScope: global processing and interaction via GlobalAudioplayersPlatformInterface
• AudioplayersPlatformInterface: interface to player platform channel (methods and events), holds an instance implementation like AudioplayersPlatform or WebAudioplayersPlatform
• GlobalAudioplayersPlatformInterface: interface to global platform channel (methods and events), holds an instance implementation like GlobalAudioplayersPlatform or WebGlobalAudioplayersPlatform
• AudioplayersPlatform: platform channel implementation of AudioplayersPlatformInterface
• GlobalAudioplayersPlatform: platform channel implementation of GlobalAudioplayersPlatformInterface

Also deprecated AudioPlayer.global.setGlobalAudioContext() in favor of AudioPlayer.global.setAudioContext() as the method is called on the global scope anyways and other methods don't have the global keyword in it.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs:, chore: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

Migration instructions

audioplayers:

Before	After
GlobalPlatformInterface	GlobalAudioScope
AudioPlayer.global.setGlobalAudioContext()	AudioPlayer.global.setAudioContext()

audioplayers_platform_interface:

Before	After
AudioplayersPlatform	AudioplayersPlatformInterface
MethodChannelAudioplayersPlatform	AudioplayersPlatform
GlobalPlatformInterface	GlobalAudioplayersPlatformInterface
MethodChannelGlobalPlatform	GlobalAudioplayersPlatform

audioplayers_web:

Before	After
AudioplayersPlugin	AudioplayersPlugin, WebAudioplayersPlatform and WebGlobalAudioplayersPlatform

Related Issues

#1352