Add additional merchandise GraphQL in CartLineFragment fields to closer match VariantFragment

[mckenzieclark]

Hello! Not sure on the status of the sd-cart branch, but it currently appears to be the only way to continue using js-buy-sdk with support for the Cart API.

I'm working on a migrating a site from using Checkout to Cart, and one thing we rely on is getting back a certain amount of line item data back from our Storefront API calls, including image URL and product title. The Checkout VariantFragment in the main branch ensured we got the data we needed. With that required data having moved into the ProductVariant object AKA the merchandise field, the CartLineFragment is now only returning an ID. I've updated this to include the values originally present in VariantFragment.

Perhaps there's another way to retrieve this data using client.graphQLClient.query but with ...CartFragment being the mainstay for all mutation return values, this doesn't seem possible.