[Refactored] add otp_text_field barrier file

[m7mdra]

Note this is a breaking change.

users will now have to remove any reference to

import 'package:otp_text_field/otp_field.dart';
import 'package:otp_text_field/style.dart';

while keeping only

import 'package:otp_text_field/otp_text_field.dart';

This update helps:

• Keep imports consistent across projects.
• Expose only the intended, public API, reducing potential misuse of internal components.