migration symfony 5 4 (#300)

* symfony 5.4 (diff dev)

* symfony 5.4 (working)

* symfony 5.4 (update autoload)

* symfony 5.4 (remove swiftmailer mailer implementation)

* symfony 5.4 (php doc and split Global accessor class)


### Impacted packages:

composer require php:">=7.2.5 <8.0.0" symfony/console:5.4.* symfony/dotenv:5.4.* symfony/framework-bundle:5.4.* symfony/twig-bundle:5.4.* symfony/yaml:5.4.* --update-with-dependencies

composer require symfony/stopwatch:5.4.* symfony/web-profiler-bundle:5.4.* --dev --update-with-dependencies

lib/symfony/string/Slugger/SluggerInterface.php

@@ -0,0 +1,27 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\String\Slugger;
+
+use Symfony\Component\String\AbstractUnicodeString;
+
+/**
+ * Creates a URL-friendly slug from a given string.
+ *
+ * @author Titouan Galopin <galopintitouan@gmail.com>
+ */
+interface SluggerInterface
+{
+    /**
+     * Creates a slug for the given string and locale, using appropriate transliteration when needed.
+     */
+    public function slug(string $string, string $separator = '-', string $locale = null): AbstractUnicodeString;
+}