Add Support for WooCommerce Multilingual multicurrency

One of the developers of the WooCommerce Multilingual project reached out to me, asking if we could add support for their plugin to WP Super Cache. This [page](https://git.onthegosystems.com/glue-plugins/wpml/woocommerce-multilingual/-/wikis/Integrate-caching-for-multicurrency) describes how to integrate it into a general caching plugin, and I think WP Super Cache would be easy to modify in that way.

It would probably best be done as a WP Super Cache plugin, so users of that plugin can activate that feature in WP Super Cache.

Or, the WooCommerce Multilingual plugin could ship their own plugin that would be copied into the WP Super Cache directory if they detected that WP Super Cache was installed.

The plugin could add the `wcml_client_currency` cookie to the rejected cookies list, to avoid caching those pages completely.
Or use the `wp_cache_get_cookies_values` cacheaction to add the cookie name/value to the cache key of the current page.

Start by reading the linked WooCommerce Multilingual multicurrency integration page, then inspect WP Super Cache's plugin extension points for the rejected cookies list and the wp_cache_get_cookies_values cache action. Done means currency-specific WooCommerce pages are handled without serving one currency's cached response to another, using one of the proposed integration approaches.

Written by the indexing model from the issue text.