FR: Enum with errors

I find useful having this
https://firebase.google.com/docs/auth/admin/errors

as an enum exported from typings.

Is this something you are fine with? I can open a PR

Start by reviewing the Firebase Authentication Admin errors reference linked in the issue and the repository's existing exported typings. Determine how the documented errors should be represented as an enum; done means the enum is available from the public typings and covers the intended error values.

索引モデルが issue の本文から書いたものです。