4.0 | .gitignore: ignore the PHPUnit cache file

As PHPCS 4.x now supports PHPUnit 8 and 9, which automatically enables caching of the test results, a `.phpunit.result.cache` file will be created.

This file should not be committed, so adding it to the `.gitignore` file.

Author: jrfnl

.gitignore

@@ -4,3 +4,4 @@
 .idea/*
 /vendor/
 composer.lock
+/.phpunit.result.cache