Better support for account linking

[alfongj]

Users sometimes want to have multiple credentials associated with the same account. NextJS makes this real hard

1. There's no documentation I could find about how to link. Reading the code, I see it's possible, but I could't figure out how
2. The AccountNotLinkedError doesn't say which provider they signed in. If it did, it would be possible to present the user with a flow where they are told: you signed in with that email and Google. Sign in with Google and later associate your Facebook with it or whathaveyou