Add UniversalPlatform.value getter

[emme1444]

This exposes the current enum value.

This is useful in order to, for example, switch upon the current platform:

import 'package:universal_platform/universal_platform.dart';

void main() {
  String platformType;
  switch (UniversalPlatform.value) {
    case UniversalPlatformType.Web:
      platformType = "Web";
      break;
    case UniversalPlatformType.Android:
    case UniversalPlatformType.Fuchsia:
    case UniversalPlatformType.IOS:
      platformType = "Mobile";
      break;
    case UniversalPlatformType.Linux:
    case UniversalPlatformType.MacOS:
    case UniversalPlatformType.Windows:
      platformType = "Desktop";
      break;
  }

  print(platformType);
}

If you want me to add the example somehow, please let me know.

Thanks!