util/error_handler library

1. yenepay_flutter package
2. documentation
3. util/error_handler.dart

yenepay_flutter package

1. Libraries
2. constants
3. models/enums
4. models/yenepay_item
5. models/yenepay_parameters
6. util/checkout_url_generater
7. util/error_handler
8. util/extensions
9. util/payment_util
10. yenepay_flutter