Define Wishlist schema

[lfades]

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Our Wishlist type is currently set to any and is defined here: framework/commerce/types/wishlist.ts. Similar to our other core types, we need to have it defined to a shape that can work with any e-commerce provider we throw at it.

Every field defined needs to have docs, we do that using comments, e.g:

/**
 * The wishlist's id.
 */
id: string