Build in state management for carts using a SWR strategy

[ScopeyNZ]

It would be nice if we could have immediate cart state when a page renders by using browser local storage to save cart state. We could also apply a "stale while revalidate" strategy, where we show the stale data from local storage, but still go and fetch the cart from the server on load to ensure data is fresh.

Additionally, we could add in functionality to set the cart as "stale" (and refresh it) when a window regains focus, meaning carts will self-update if a customer has added items to their cart on a different tab.