* first cut - removing an item from cart:
- add actions to cart store for removing an item and keeping track of
pending removal API call
- add reducer logic for storing pending state on an item, and removing
an item
- expose removeCartItem on new useStoreCartItems hook
- hook it up to remove link / trashcan icon in row item
* disable cart quantity picker/remove link while API request in progress:
- expose cart item pending status from store using selector
- use selector to disable quantity related components in line item row
* fix jsdoc - getCartItem returns undefined if not found
* add typedef for cart items store object provided by hook
* fix rebase error - key prop went awol
* orient useStoreCartItem hook to single cart item:
- simplify interface for client component
- isPending bool (was callback)
- removeItem callback no need to specify item key
+ reinstate disabled prop on remove link when updating (lost in rebase)
* move cart item pending state out of cartItems, preserve API state shape:
- pending is now stored as array of keys
- fix isItemQuantityPending selector (now much simpler)
* ensure react knows that our useSelect depends on cartItemKey
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