fix(router): Can return 'handler' param in `'route', $identifier` hook again

In 1.8, it was possible to delegate to a different page handler by registering
a callback for the `'route', $identifier` plugin hook that looks like so:

```php
$result['handler'] = 'other';
return $result;
```

Then during 1.9 it was changed to expect the key to be 'identifier':

```php
$result['identifier'] = 'other';
return $result;
```

This change makes it so that both approaches will work as intended.

Fixes #6696
This isn't going to work. We need to examine both *for changes*. The hook $value will receive both, and in practice most hook handlers will change the array and return it as is, so the hook's return will likely *always* have both keys.