feat(cart): add cart links that fill the cart and open checkout

[Rl0007]

1. A shop could not send a shopper a link that fills their cart. A campaign link landed on a product page, and the shopper had to pick sizes and quantities again.
2. Now a link like /cart/<item code>:<qty>,<item code>:<qty>?discount=CODE fills the cart and opens checkout. The discount is applied, and checkout[email] / checkout[shipping_address][city] (and the other address fields) fill in empty checkout fields. UTM parameters carry through. When guest checkout is off, the link stops at the cart and continues to checkout after sign-in.
3. To make one, fill a cart on the storefront and press Copy cart link on the cart or checkout page. The link carries the cart's lines and its discount code, so shoppers can share their cart too.

[NagariaHussain]

Have you seen any e-commerce platform showing copy cart link? 👀