Allow CssXPath updates with composer (#4368)

* Install CssXPath with composer

* Fix code style in tests

* fix pathnames for tests and linting

* add irrelevant files to .gitignore

* Alphabetic order

* let composer sort the packages alphabetically

Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

lib/phpgt/cssxpath/src/CssXPathException.php

@@ -0,0 +1,6 @@
+<?php
+namespace Gt\CssXPath;
+
+use RuntimeException;
+
+class CssXPathException extends RuntimeException {}