Add support of dist file name phpcs.dist.xml

To achieve a better editor experience, XML files should have a
corresponding extension, as this is often used to recognize the file
type and derive corresponding features such as highlighting.

src/Config.php

@@ -397,6 +397,8 @@ public function __construct(array $cliArgs=[], $dieOnUnknownArg=true)
                 'phpcs.xml',
                 '.phpcs.xml.dist',
                 'phpcs.xml.dist',
+                '.phpcs.dist.xml',
+                'phpcs.dist.xml',
             ];
 
             do {