Consider declaring `strict_types`

I noticed the Abilities API landed in core [with strict types](https://github.com/WordPress/wordpress-develop/blob/d015d16c1f165588efb481aa9b51675bac02264a/src/wp-includes/abilities-api/class-wp-ability.php#L12):

```php
declare( strict_types = 1 );
```

This should be easy for us to turn on for Performance Lab since we've already implemented PHPStan level 8.