Function onAuthStateChange

onAuthStateChange(callback: OnAuthStateChangeFn): void
Sets a callback function to be called when the authentication state changes, i.e. when a user signs in or out.
Parameters
- callback: OnAuthStateChangeFn
  The callback function to be called when the authentication state changes.
Returns void
Example
```
onAuthStateChange(async (user) => {
  if (user) {
    console.log("User signed in:", user.userId);
  } else {
    console.log("User signed out");
  }
});
```