[CORE][EXTENSIONS] Added extension (modules, plugins) loading and test plugin, which is able to handle events

plugins/available/Test/Test.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace Plugin\Test;
+
+class Test
+{
+    public function onTest(string $foo)
+    {
+        dump('Event handled: ' . $foo);
+    }
+}