feat: enable Shopify checkout from cart

[NWRichmond]

WHY are these changes introduced?

Addresses LD-1236

Add @nacelle/shopify-checkout to reference store

Before this PR, clicking the cart's Checkout button didn't create a checkout.

WHAT is this pull request doing?

Adds Shopify checkout functionality from @nacelle/shopify-checkout.

How to Test

• Go to https://nuxt-reference-store-git-ld-1236-checkout-from-cart-nacelle.vercel.app
• Add some items to the cart and perform a checkout
  • the checkoutId should be saved to localStorage
  • if the checkout isn't completed, the same checkoutId should be reused, even if new products are added to the cart in between checkouts
  • if the checkout is completed, the cart should reset

Demonstration

Creating a checkout

demo-reference-store-checkout.mov

A completed checkout clears the cart

demo-reference-store-checkout-completed.mov

[vercel]

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🔍 Inspect: https://vercel.com/nacelle/nuxt-reference-store/DM6FRtDvWQJwrn1rFTnhkchgfBtB
✅ Preview: https://nuxt-reference-store-git-ld-1236-checkout-from-cart-nacelle.vercel.app

[jasonredick]

Works great 👍

Not a needed change, but I wonder if we should prevent cart actions from happening while isCheckingOut is true, things like adding a cross sell to the cart or removing an item from the cart.

[NWRichmond]

...I wonder if we should prevent cart actions from happening while isCheckingOut is true, things like adding a cross sell to the cart or removing an item from the cart.

Good idea, @jasonredick! Implemented in 9e0507f:

demo-block-cart-actions-while-checking-out.mov

[github-actions]

🎉 This PR is included in version 1.2.0 🎉

The release is available on GitHub release

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