Update coding style to include PSR-12, among other options

This also includes heavy use of slevomat/coding-standard to apply
various checks to the code, based on maintainer (me) preference.

src/Exception/InvalidUuidStringException.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * This file is part of the ramsey/uuid library
  *
@@ -7,11 +8,10 @@
  *
  * @copyright Copyright (c) Ben Ramsey <ben@benramsey.com>
  * @license http://opensource.org/licenses/MIT MIT
- * @link https://benramsey.com/projects/ramsey-uuid/ Documentation
- * @link https://packagist.org/packages/ramsey/uuid Packagist
- * @link https://github.com/ramsey/uuid GitHub
  */
 
+declare(strict_types=1);
+
 namespace Ramsey\Uuid\Exception;
 
 use InvalidArgumentException;