Add impersonator support

[nicknisi]

Summary

• Adds impersonator field to the useAuth hook
• Exposes impersonator email and reason when a session is being impersonated
• Exports Impersonator type for TypeScript users

Fixes #68

[greptile-apps]

Greptile Summary

This PR adds impersonator support to the AuthKit React library, bringing feature parity with other WorkOS client SDKs. The changes introduce a new Impersonator type containing email (string) and reason (string | null) fields that gets exposed through the useAuth hook when a session is being impersonated.

The implementation follows the existing authentication state management patterns in the codebase:

1. Type Definition: The Impersonator interface is defined in state.ts and exported through index.ts for public consumption
2. State Integration: The provider extracts impersonator data from the OnRefreshResponse in the handleRefresh callback and includes it in state updates
3. Session Comparison: Impersonator fields are properly included in the session equality checks to ensure state updates trigger correctly when impersonation status changes
4. Context Exposure: The impersonator data flows through the authentication context and becomes available via the useAuth hook

The changes maintain backward compatibility by making the impersonator field nullable with a default value of null. The implementation enables React applications to build impersonator components similar to those available in the Next.js SDK, addressing the functionality requested in issue #68.

Confidence score: 5/5

• This PR is safe to merge with minimal risk as it adds new functionality without breaking existing behavior
• Score reflects well-structured implementation following established codebase patterns with proper null handling and backward compatibility
• No files require special attention as the changes are straightforward additions to the existing authentication flow

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