Add useCartElement and useCartElementState hooks

[martinalong-stripe]

Summary & motivation

useCartElement
useCartElement is a react hook that retrieves the Cart Element, similar to the effect of calling elements.getElement('cart'). This hook exists so that React users can more easily call cart functions such as:

const cartElement = useCartElement();

cartElement.addLineItem({product: "prod_123"});
cartElement.show();
cartElement.hide();
cartElement.cancelCheckout("Checkout cancelled because...");
cartElement.update({ showOnAdd: false );

useCartElementState
useCartElementState is a react hook that retrieves the last value returned by the ready or change events (or null if no event has occurred yet). This value is used to display the cart count in the merchant's UI. The value returned has the shape:

{
  id: "cart_session_123",
  elementType: 'cart',
  lineItems: {
    count: 1,
  }
}

Testing & documentation

Wrote unit tests, and a ran a storybook scenario to confirm it works