storePaymentPurposePremiumSubscription property
The user subscribed to Telegram Premium
Implementation
StorePaymentPurposePremiumSubscription?
    get storePaymentPurposePremiumSubscription {
  if (this is StorePaymentPurposePremiumSubscription) {
    return this as StorePaymentPurposePremiumSubscription;
  }
  return null;
}