add chapter 12 solutions

implementation/12/rector.php

@@ -0,0 +1,12 @@
+<?php declare(strict_types=1);
+
+use Rector\Config\RectorConfig;
+use Rector\Set\ValueObject\LevelSetList;
+
+return static function (RectorConfig $rectorConfig): void {
+    $rectorConfig->paths([__DIR__ . '/src', __DIR__ . '/config', __DIR__ . '/ecs.php', __DIR__ . '/rector.php']);
+
+    $rectorConfig->importNames();
+
+    $rectorConfig->sets([LevelSetList::UP_TO_PHP_81]);
+};