Adds and applies a coupon to the cart.
について applyCoupon() function returns a Promise that resolves to the current
site visitor's cart when the specified coupon has been applied.
The cart can only hold one coupon. If a coupon is already applied to the cart, passing a different couponCode to applyCoupon() will replace the existing one.
注： When editing a site as a contributor, applyCoupon() will only work when viewing the live site.
The code of the coupon to be applied to the cart.