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/product/add-to-cart.tsx

@@ -0,0 +1,74 @@
+'use client';
+
+import { useRouter, useSearchParams } from 'next/navigation';
+import { useEffect, useState, useTransition } from 'react';
+
+import LoadingDots from 'components/loading-dots';
+import { ProductVariant } from 'lib/shopify/types';
+
+export function AddToCart({
+  variants,
+  availableForSale
+}: {
+  variants: ProductVariant[];
+  availableForSale: boolean;
+}) {
+  const [selectedVariantId, setSelectedVariantId] = useState(variants[0]?.id);
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const [isPending, startTransition] = useTransition();
+  const [adding, setAdding] = useState(false);
+
+  useEffect(() => {
+    const variant = variants.find((variant: ProductVariant) =>
+      variant.selectedOptions.every(
+        (option) => option.value === searchParams.get(option.name.toLowerCase())
+      )
+    );
+
+    if (variant) {
+      setSelectedVariantId(variant.id);
+    }
+  }, [searchParams, variants, setSelectedVariantId]);
+
+  const isMutating = adding || isPending;
+
+  async function handleAdd() {
+    if (!availableForSale) return;
+
+    setAdding(true);
+
+    const response = await fetch(`/api/cart`, {
+      method: 'POST',
+      body: JSON.stringify({
+        merchandiseId: selectedVariantId
+      })
+    });
+
+    const data = await response.json();
+
+    if (data.error) {
+      alert(data.error);
+      return;
+    }
+
+    setAdding(false);
+
+    startTransition(() => {
+      router.refresh();
+    });
+  }
+
+  return (
+    <button
+      aria-label="Add item to cart"
+      onClick={handleAdd}
+      className={`${
+        availableForSale ? 'opacity-90 hover:opacity-100' : 'cursor-not-allowed opacity-60'
+      } flex w-full items-center justify-center bg-black p-4 text-sm uppercase tracking-wide text-white dark:bg-white dark:text-black`}
+    >
+      <span>{availableForSale ? 'Add To Cart' : 'Out Of Stock'}</span>
+      {isMutating ? <LoadingDots className="bg-white dark:bg-black" /> : null}
+    </button>
+  );
+}