[Quick Add] Quantity Picker for Quick Add *Enhancement*

[bredowmax]

Describe the enhancement you'd like
Quantity Picker for Quick add.

With #1388, Shopify introduced a Quick Add feature that allows adding products from a product grid - a very important improvement. This, however, doesn't consider a use case where the shopper wants 3x Carrot and 5x Paprika.

For stores with a large inventory and low-consideration products (such as Food, Grocery), the number of clicks required to buy a large basket is one of the most important decision factors when choosing a theme.

This feature would help tailor the Taste & Crave themes to their respective niches.

How it works

1. On first click of a product with only 1 variant, product is added to the cart. At the same time, a quantity picker is revealed.
2. On the right hand side of that quantity picker, shopper can increase quantity step by step
3. On the left hand side, shopper can decrease quantity. If current quantity in the cart is 1, that means that the product will be deleted from the cart
4. After a delay of a few seconds, the quantity picker disappears and is replaced by the current number of products in the cart (optional)

Describe alternatives you've considered
There is the Modal option of Quick Add - but this results in unnecessary clicks & loading time. Grocery shoppers do not want to read more about the product - they want to be able to choose the desired quantity with as few clicks as possible.

Many native mobile apps do offer this feature. Of the official themes in the Shopify theme store, I'm only aware of the Canopy theme that supports this feature. Empire, Warehouse & Superstore do not support this feature last time I checked.

Additional context/screenshots
Screenshots from Walmart & Instacart apps wi