add invert scan feature for android

[akkalbhor]

This PR fixes #403
@juliuscanute Please review and merge ASAP

[juliansteenbakker]

You need to add an empty function for flutter_qr_web like so:

  @override
  Future<void> scanInvert() {
    // TODO: implement scanInvert
    throw UnimplementedError();
  }

[akkalbhor]

I have added the function in flutter_qr_web

[akkalbhor]

@juliansteenbakker can you merge this PR and update the library as well?

[akkalbhor]

@juliansteenbakker can you please update the library on pub.dev?

[JT501]

please upload a new version to pub.dev, we need this fix. 🙏🏻