fix: export user.dart to expose OSUserChangedState

[nan-li]

Description

One Line Summary

Export user.dart to expose OSUserChangedState.

Details

Motivation

• Fixes [Bug]: OSUserChangedState is not publicly exported, preventing type-safe use of OneSignal.User.addObserver #1049
• Checked other files that are not exported, and they don't need to be. They have no other types to expose.

Scope

Export user.dart

Testing

Unit testing

NA

Manual testing

Reproduced by adding typing of OSUserChangedState to our example app, which errors before changes in this PR.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes - only exposing a type meant to be public

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is