[local_auth] Add missing error codes that used in version 1.6.0

[amir14a]

Adds missing error codes to error_codes.dart that were used during the development of version 1.6.0.

In version 1.6.0 of the local_auth package, two new error codes were added and used in the package's implementation; however, they were not added to the error_codes.dart file. Therefore, users cannot use them unless they are hardcoded.
This PR is added them to the error_codes.dart.
Relative links:

• packages/packages/local_auth/local_auth_darwin/CHANGELOG.md

  Line 9 in 3dae1f6

  * `UserCancelled` is now returned when the user cancels the prompt.

• packages/packages/local_auth/local_auth_darwin/lib/local_auth_darwin.dart

  Line 84 in 3dae1f6

  code: 'UserCancelled',

• packages/packages/local_auth/local_auth_darwin/lib/local_auth_darwin.dart

  Line 96 in 3dae1f6

  code: 'UserFallback',

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[gemini-code-assist]

Code Review

This pull request correctly adds two missing error codes, userCancelled and userFallback, to the local_auth package. These were already in use within the local_auth_darwin implementation, and this change makes them available to developers without hardcoding. My review includes a suggestion to improve the accuracy of the documentation for these new codes.