Cart Provider should be able to initalize cart if not found

[hsdonkin]

Would love to be able to pass a flag into CartProvider like onLoadCreate!

I'm running into a weird situation where I need to send an add to cart event to analytics. When the line item is added with linesAdd, the cart technically doesn't exist yet, so id from useCart() is undefined.

It would be much easier if I could guarantee a cart exists before the form is submitted. I've tried a workaround where I call cartCreate() if cartReady and no cartId, but it creates some other bugs in our app where the onLinesAdd callbacks get triggered by cartCreate()