The JS Buy SDK now matches the updated schema for the Storefront API

We're updating the JS Buy SDK so that it matches the new Storefront API schema.

This update does the following:

• Removes unused fixtures.
• Allows collections, products, collectionByHandle, and productByHandle to be fetched from the QueryRoot instead of from the Shop object.
• Returns checkoutUserErrors when they're present (instead of returning userErrors). This adds an extra field called code.
• Exposes the checkoutLineItemsReplace mutation (to replace the deprecated checkoutLineItemsAdd , checkoutLineItemsRemove , and checkoutLineItemsUpdate mutations).

You can learn more about these changes here.