Update cartAdd to support selling_plan_id parameter for subscriptions / selling plans

[sebszocinski]

As title suggests, the cartAdd endpoint doesn't support the selling_plan_id for adding a subscription product to cart.

See here: https://shopify.dev/docs/themes/ajax-api/reference/cart#add-a-selling-plan

[leighgibbo]

Bumping this issue in celebration of the 1 year anniversary of the above PR - any chance we can get this one merged and closed? @chrisberthe @t-kelly @tauthomas01

[bertiful]

Happy anniversary!

FYI two out of the three people you pinged no longer work for Shopify, and the remaining person no longer works on the Themes team. And given the approach that Shopify took with Dawn, this repo is most likely considered dead. @ludoboludo? :)

[ludoboludo]

Hey folks 👋 @chrisberthe is right. With the recent shift in approach towards how we build themes, this is a depreciated repo that isn't receiving any updates.