Adds a check for index.php in the web/ directory.

If a `web/index.php` file does not exist, it will be created automatically during composer install. Resolves an issue that occurs when that file is missing and the site fails to load.

For more information, see https://docs.pantheon.io/release-notes/2025/02/wordpress-composer-managed-1-32-4

Author: jazzsequence