add FileCheck

[david-d-h]

This PR adds the FileCheck, as the name suggests, it will check if the configured files exist. It can be configured like this

OK::checks([
    FileCheck::config()
        ->files([
            'routes/web.php',
            'routes/api.php',
        ]),
]);

If a configured path starts with / it will be relative to the root of the filesystem. Otherwise it will be relative to the root of your project (via the base_path() helper in Laravel).