Longcache should purge site cache when custom CSS is updated

Our client has reported that when updating a site's Custom CSS. They incorrectly interpret this as updating a page. However, the default behaviour of Longcache probably should include a site purge when Custom CSS has been updated.

It has been many a year since I've touched anything related to Custom CSS, but it looks as though [this hook](https://developer.wordpress.org/reference/hooks/update_custom_css_data/) is the best way to achieve this.