PHP Fatal if `$query->get( 'order' )` is an array

### Description of the bug

In https://github.com/alleyinteractive/searchpress/blob/b90a875a6c11f7b83e189bd8e039043ff6cf8703/lib/class-sp-integration.php#L297-L303 the `order` value of the query gets passed directly to `strtolower()`. It's possible for the [order parameter of WP_Query](https://developer.wordpress.org/reference/classes/wp_query/#order-orderby-parameters) to be an array and SearchPress currently can't handle that.

### Steps To Reproduce

Run query like this:

```
$query = new WP_Query([
'orderby' => [ 'title', 'date' ],
'order' => [ 'DESC', 'ASC' ],
]);
```

### Additional Information

_No response_