feat!: Extend AudioContextConfig.duckAudio to AudioContextConfig.focus

[Gustl22]

Description

• Introduces AudioContextConfigFocus.mixWithOthers to unify AVAudioSessionOptions.mixWithOthers (iOS) and AndroidAudioFocus.none (Android)
• Replaces the option AudioContextConfig.duckAudio = true with AudioContextConfig.focus = AudioContextConfigFocus.duckOthers

Closes #1718

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, refactor:,
  docs:, chore:, test:, ci: 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

Before:

AudioContextConfig(
  duckAudio: true,
);

After:

AudioContextConfig(
  focus: AudioContextConfigFocus.duckOthers,
);

Related Issues

Closes #1718

[spydon]

Lgtm!