FastStore: Keep split cart lines with `preserveCartLines`

FastStore now supports the opt-in api.preserveCartLines flag, which keeps the cart lines of the same SKU that your store split in Checkout separate.

By default, FastStore merges cart lines with the same SKU into a single line. When a store splits a line in Checkout using the split item endpoint, for example, [refrigerator x1, refrigerator x1], FastStore merges them into [refrigerator x2] on the next cart validation, undoing the split in Checkout.

With preserveCartLines: true, FastStore respects the lines created in Checkout. Each line is shown separately in the minicart, and editing or removing a line affects only that line. FastStore doesn't split lines on its own, and stores that don't enable the flag keep the current behavior.

Some stores need each unit to be a separate line so shoppers can choose different assembly options for each unit, or so each unit can be integrated individually with an ERP.

Update your store to FastStore v4.10.3 or later, and set preserveCartLines to true in the api object of your discovery.config.js file. For step-by-step instructions, see Keeping split cart lines. For the full list of options, see Configuration options for discovery.config.js.