countryCode method
Returns the user's country.
Android: Returns Play billing country code based on ISO-3166-1 alpha2 format.
See: https://developer.android.com/reference/com/android/billingclient/api/BillingConfig See: https://unicode.org/cldr/charts/latest/supplemental/territory_containment_un_m_49.html
iOS: Returns the country code from SKStoreFrontWrapper.
See: https://developer.apple.com/documentation/storekit/skstorefront?language=objc
Implementation
Future<String> countryCode() => InAppPurchasePlatform.instance.countryCode();