Add grammar constraint for phpcs.xml.dist (#572)

This should enhance the DX for this file.

Author: greg0ire

phpcs.xml.dist

@@ -1,5 +1,8 @@
 <?xml version="1.0"?>
-<ruleset>
+<ruleset
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="vendor/squizlabs/php_codesniffer/phpcs.xsd"
+>
     <arg name="basepath" value="."/>
     <arg name="extensions" value="php"/>
     <arg name="parallel" value="80"/>