* Adds a `isSupportedSiteURL` method to the `ShoppingProduct` to determine if a URL is on a supported shopping site. * Passes isProductPage and isSupportedSite booleans to the shopping container to render the empty state UIs * Switches to checking if shopping container pref boolean properties is not nullish before setting in `_update`. This cleaned up the update objects in `ShoppingSidebarChild` and made adding the new empty states simpler. Differential Revision: https://phabricator.services.mozilla.com/D227752