Capture initialized notification

https://microsoft.github.io/language-server-protocol/specification#initialized

src/LanguageServer.php

@@ -292,6 +292,16 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
         });
     }
 
+    /**
+     * The initialized notification is sent from the client to the server after the client received the result of
+     * the initialize request but before the client is sending any other request or notification to the server.
+     * The server can use the initialized notification for example to dynamically register capabilities.
+     * The initialized notification may only be sent once.
+     */
+    public function initialized() {
+        // Do nothing
+    }
+
     /**
      * The shutdown request is sent from the client to the server. It asks the server to shut down, but to not exit
      * (otherwise the response might not be delivered correctly to the client). There is a separate exit notification that