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/error-handler/Debug.php

@@ -0,0 +1,41 @@
+<?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\ErrorHandler;
+
+/**
+ * Registers all the debug tools.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+class Debug
+{
+    public static function enable(): ErrorHandler
+    {
+        error_reporting(-1);
+
+        if (!\in_array(\PHP_SAPI, ['cli', 'phpdbg'], true)) {
+            ini_set('display_errors', 0);
+        } elseif (!filter_var(ini_get('log_errors'), \FILTER_VALIDATE_BOOLEAN) || ini_get('error_log')) {
+            // CLI - display errors only if they're not already logged to STDERR
+            ini_set('display_errors', 1);
+        }
+
+        @ini_set('zend.assertions', 1);
+        ini_set('assert.active', 1);
+        ini_set('assert.warning', 0);
+        ini_set('assert.exception', 1);
+
+        DebugClassLoader::enable();
+
+        return ErrorHandler::register(new ErrorHandler(new BufferingLogger(), true));
+    }
+}