Using `linkWithCredential()`

Understanding Account Linking
Firebase allows a user to associate multiple authentication methods (email, Google, Facebook, etc.) with a single account. This avoids duplicates and improves the user experience.

Using linkWithCredential()
The primary method for linking an account is linkWithCredential(). It allows you to add a new provider to an already connected account. For example, linking Facebook to an existing email account.

Example of Linking with Google
If the user is logged in with email/password, the developer can initiate a Google login, retrieve the GoogleAuthCredential, and then call linkWithCredential() on the current user.