[WordPress] Suggest the Performance Lab plugin where applicable

A number of WordPress-specific audit messages include a link to the search results page at wordpress.org/plugins. Now that the WordPress Performance working group has built the [Performance Lab](https://wordpress.org/plugins/performance-lab/) plugin, I'd like to see Lighthouse making a more opinionated recommendation to use that specific plugin where applicable.

For example, the `modern-image-formats` audit has a WordPress stack pack message of:

> Consider using a [plugin](https://wordpress.org/plugins/search/convert+webp/) or service that will automatically convert your uploaded images to the optimal formats.

The Performance Lab plugin promotes a module that does exactly that:

> **WebP Uploads:** Creates WebP versions for new JPEG image uploads if supported by the server.

cc @adamsilverstein @felixarntz @thierrya