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/psr/event-dispatcher/src/StoppableEventInterface.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+namespace Psr\EventDispatcher;
+
+/**
+ * An Event whose processing may be interrupted when the event has been handled.
+ *
+ * A Dispatcher implementation MUST check to determine if an Event
+ * is marked as stopped after each listener is called.  If it is then it should
+ * return immediately without calling any further Listeners.
+ */
+interface StoppableEventInterface
+{
+    /**
+     * Is propagation stopped?
+     *
+     * This will typically only be used by the Dispatcher to determine if the
+     * previous listener halted propagation.
+     *
+     * @return bool
+     *   True if the Event is complete and no further listeners should be called.
+     *   False to continue calling listeners.
+     */
+    public function isPropagationStopped() : bool;
+}