use Validator;

Validator::extend()を使う時はuse Validator;を呼び出すのがポイント

環境

$ php artisan --version
Laravel Framework 5.7.19

/var/www/vhosts/example.net/httpdocs/project/app/Providers/HelloServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\View;
use Illuminate\Support\ServiceProvider;
use Illuminate\Validation\Validator;
use App\Http\Validators\HelloValidator;


class HelloServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap services.
     *
     * @return void
     */
    public function boot()
    {
        Validator::extend('hello', function($attribute, $value, $parameters, $validator){
           return $value % 2 == 0;
        });
    }

}

Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_ERROR)
Call to undefined method Illuminate\Validation\Validator::extend()

エラー発生

/var/www/vhosts/example.net/httpdocs/project/app/Providers/HelloServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\View;
use Illuminate\Support\ServiceProvider;

//use Illuminate\Validation\Validator; ←無効化
use App\Http\Validators\HelloValidator;
use Validator; ←追加

class HelloServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap services.
     *
     * @return void
     */
    public function boot()
    {
        Validator::extend('hello', function($attribute, $value, $parameters, $validator){
           return $value % 2 == 0;
        });
    }

}

表示出来た