Laravel Scout support

[Cellard]

This PR brings support of Laravel\Scout\Builder to FromQuery concern.

Users not infrequently want to export their search results. Laravel Scout is a good instrument for searching.

namespace App\Exports;

use App\Invoice;
use Maatwebsite\Excel\Concerns\FromQuery;

class InvoicesExport implements FromQuery
{
    public function query()
    {
        return Invoice::search('Search phrase');
    }
}

I write tests — for queued and direct writer.

• Checked the codebase to ensure that your feature doesn't already exist.
• Take note of the contributing guidelines.
• Checked the pull requests to ensure that another person hasn't already submitted a fix.
• Added tests to ensure against regression.

[joelharkes]

@Cellard this breaks for everyone not using Laravel scout but do use laravel Excel :(

[patrickbrouwers]

Didn't break anything for me. I'm not using Laravel Scout, everything still works like expected. instanceof checks can run on non-existing classes.