Firebase errors are exported as stubs

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### [REQUIRED] Step 2: Describe your environment

* Operating System version: _____
* Firebase SDK version: 12.1.1
* Firebase Product: auth
* Node.js version: _____
* NPM version: _____

### [REQUIRED] Step 3: Describe the problem

#### Steps to reproduce:

The new error types that are exported in 12.1.1 are great and make error handling much easier. However, the TS interfaces are notably lacking constructors - they just extend all the way to Error. The _actual_ implementations take `info: ErrorInfo, message?: string` parameters to set the underlying `code` and `message` properties.

This means that in testing code I'm not able to really take advantage of the new types without casting or playing other type shenanigans in order to set the properties as expected, which significantly limits the usefulness of these newly exported types.

#### Relevant Code:

```
// In a node REPL

> let err = new FirebaseAuthError("test error message")
undefined
> err.code
'auth/undefined'
> err.message
undefined

> let err2 = new FirebaseAuthError({ message: "test error 2", code: "val" })
undefined
> err2.code
'auth/val'
> err2.message
'test error 2'

// In typescript
// Argument of type '{ message: string; code: string; }' is not assignable to parameter of type 'string'.
const error = new FirebaseAuthError({ message: "test error 2", code: "val" });
```

Relates to https://github.com/firebase/firebase-admin-node/pull/2549