fix(auth, web): get auth credential from exception and pass to user if one is available

[russellwheatley]

Description

I tested this with user's example and it worked as intended. The JS error is correctly parsed and instantiated as a Dart credential if it exists. If not, null is passed as credential which is a nullable argument on FirebaseAuthException:

final AuthCredential? credential;

Related Issues

fixes https://github.com/firebase/flutterfire/issues/12771

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Breaking Change

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[mertguler94]

Waiting for this PR to be merged, thanks @russellwheatley .