Allow components to _either_ use context provider or use props

[frehner]

Quite a few components rely on a context provider and associated hook, but in many cases it may also make sense to allow devs to pass in a prop instead and not rely on setting up a context provider.

For example, <Metafield/> uses useShop(), but only need the locale from it. We could also allow a locale prop and not rely on the useShop() hook in that case.

Tasks

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1. Metafield
2. useMoney
3. ShopPayButton

[frehner]

• AddToCartButton uses useCart(), useProduct()
• BuyNowButton uses useCart()
• CartCheckoutButton uses useCart()
• CartCost uses useCart()
• Metafield uses useShop()
• ShopPayButton uses useShop()
• useMoney() uses useShop()

[frehner]

Hm, while it makes sense for easy ones like locale to be either a prop or from context, I'm less sure on complex ones, like those that come from the CartProvider.

[elisenyang]

prop should override context

if no prop value is provided - we default to context

[frehner]

We should also update CartProvider to optionally pull from ShopifyConfig the contryCode.

[frehner]

Decision: leave the useCart() things alone for now, as there is more complexity there, but allow places that use useShop() to optionally take in a prop.