[9.x] Implement runwayLinkFieldtype query scope

[duncanmcclean]

This pull request implements support for a runwayLinkFieldtype query scope, which is applied when resolving a model from the Link fieldtype.

If you need to change which models can be linked to, you may add a runwayLinkFieldtype query scope to your model:

use Illuminate\Database\Eloquent\Attributes\Scope;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Model;

class Product extends Model
{
    #[Scope]
    protected function runwayLinkFieldtype(Builder $query): void
    {
        $query->where('is_active', true);
    }
}

This works in the same way as the existing runwayListing and runwayRoutes scopes.

This PR also fixes PublishedModelsControllerTest leaving revisions behind in tests/__fixtures__/revisions. This was happening because it was overriding statamic.revisions.path, but revisions are now written via the Stache store, whose directory is configured in TestCase. I've fixed it by deleting the fixtures directory on tear down, like the other revision tests do.

Related: #849