Next.js Commerce refresh. (#966)

We're making some updates to Next.js Commerce. Everything prior to this commit marks what we're calling [`v1`](https://github.com/vercel/commerce/releases/tag/v1) as a point in time to be able to reference and still use going into the future. The current architecture of Commerce is a multi-vendor, interoperable solution, including:

- [Shopify](https://shopify.vercel.store/)
- [Swell](https://swell.vercel.store/)
- [BigCommerce](https://bigcommerce.vercel.store/)
- [Vendure](https://vendure.vercel.store/)
- [Saleor](https://saleor.vercel.store/)
- [Ordercloud](https://ordercloud.vercel.store/)
- [Spree](https://spree.vercel.store/)
- [Kibo Commerce](https://kibocommerce.vercel.store/)
- [Commerce.js](https://commercejs.vercel.store/)
- [SalesForce Cloud Commerce](https://salesforce-cloud-commerce.vercel.store/)

All features can be toggled on or off, and it's easy to change between commerce providers. To support this, we needed to create a ["commerce metaframework"](d1d9e8c434/packages/commerce/new-provider.md) where providers could confirm to an API spec to add support for Next.js Commerce. While this worked and was successful for `v1`, we have different design goals and ambitions for `v2`.

**What You Need To Know**

- `v1` will not be updated moving forward. If you need to reference `v1`, you will still be able to clone and deploy the version tagged at this release.
- `v2` will be shifting to be a single provider vs. provider agnostic. Other providers are welcome to fork this repository and swap out the underlying `lib/` implementation that connects to the selected commerce provider (Shopify). This architecture was chosen to reduce the surface area of the codebase, remove the intermediate metaframework layer for provider-interoperability, and enable usage with the latest Next.js and React features.
- We will be sharing more about `v2` in the future as we continue to iterate before the marked release.

components/cart/edit-item-quantity-button.tsx

@@ -0,0 +1,62 @@
+import { useRouter } from 'next/navigation';
+import { startTransition, useState } from 'react';
+
+import MinusIcon from 'components/icons/minus';
+import PlusIcon from 'components/icons/plus';
+import type { CartItem } from 'lib/shopify/types';
+import LoadingDots from '../loading-dots';
+
+export default function EditItemQuantityButton({
+  item,
+  type
+}: {
+  item: CartItem;
+  type: 'plus' | 'minus';
+}) {
+  const router = useRouter();
+  const [editing, setEditing] = useState(false);
+
+  async function handleEdit() {
+    setEditing(true);
+
+    const response = await fetch(`/api/cart`, {
+      method: type === 'minus' && item.quantity - 1 === 0 ? 'DELETE' : 'PUT',
+      body: JSON.stringify({
+        lineId: item.id,
+        variantId: item.merchandise.id,
+        quantity: type === 'plus' ? item.quantity + 1 : item.quantity - 1
+      })
+    });
+
+    const data = await response.json();
+
+    if (data.error) {
+      alert(data.error);
+      return;
+    }
+
+    setEditing(false);
+
+    startTransition(() => {
+      router.refresh();
+    });
+  }
+  return (
+    <button
+      aria-label={type === 'plus' ? 'Increase item quantity' : 'Reduce item quantity'}
+      onClick={handleEdit}
+      disabled={editing}
+      className={`${editing ? 'cursor-not-allowed' : ''} ${
+        type === 'minus' ? 'ml-auto' : ''
+      } flex h-8 w-8 items-center justify-center border-l border-black/40 bg-black/0 hover:bg-black/10 dark:border-white/40 dark:bg-white/0 dark:hover:bg-white/10`}
+    >
+      {editing ? (
+        <LoadingDots className="bg-white dark:bg-black" />
+      ) : type === 'plus' ? (
+        <PlusIcon className="h-4" />
+      ) : (
+        <MinusIcon className="h-4" />
+      )}
+    </button>
+  );
+}