Is your feature request related to a problem? Please describe.
Need to detect if an error is an instanceof FirebaseError

Describe the solution you'd like
Add error utils to module exports so can be used without the error below

Additional context
Currently when import { FirebaseError } from 'firebase-admin/lib/utils/error' get the following error Error: Package subpath './lib/utils/error' is not defined by "exports" in /Users/.../node_modules/firebase-admin/package.json