Anonymous account
register as anonymous
signInAnonymously(auth)
check if the account is anonymous
the account is anonymous if there is no provider data:
if (auth.currentUser?.isAnonymous)
// auth.currentUser?.providerData.length === 0
convert to an email account
we build an email credential, then link it it to the account.
// 1. build an email credential
const credential = EmailAuthProvider.credential(email, password)
// 2. link the credential to the existing account
linkWithCredential(auth.currentUser, credential)