Feature/null safety

[iuriegutu95]

Add null safety + Wav support & v2 embedding.

[ZaraclaJ]

Thanks for your contribution, i will take time to review it

[WiRight]

Any progress?

[tuanbs]

@iuriegutu95 I tried your PR and got the build error:

/Applications/flutter/.pub-cache/git/audio_recorder-79ff1153d7e8047e3f764c6c6f8d16742ca58866/ios/Classes/SwiftAudioRecorderPlugin.swift:39:53: error: 'setCategory(_:with:)' has been renamed to 'setCategory(_:options:)'
                    try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, with: AVAudioSessionCategoryOptions.defaultToSpeaker)
                                                        ^~~~~~~~~~~                                      ~~~~
                                                        setCategory                                      options
    AVFAudio.AVAudioSession:13:15: note: 'setCategory(_:with:)' was obsoleted in Swift 4.2
        open func setCategory(_ category: AVAudioSession.Category, with options: AVAudioSession.CategoryOptions = []) throws

...
Could not build the application for the simulator.
Error launching application on iPhone 8.

[tuanbs]

@iuriegutu95 I've just committed your new changes and see it works now. Thanks a lot. @ZaraclaJ Can you please merge his PR when you have chance? Thanks in advance.

[GauravCalidig]

https://pub.dev/packages/audio_recorder_nullsafety

[tuanbs]

https://pub.dev/packages/audio_recorder_nullsafety

I got some exceptions with the start and stop methods using this one.