PDP Add to cart button should always be active unless out of stock

[agurumurthy96]

As a shopper, I need the "Add to Cart" button to be always active so that I'm not guessing what I have to do to add a product to cart.

Acceptance Criteria:
"Add to Cart button" is always active (unless the item is out of stock)
When the "Add to Cart" button is clicked but the product variants haven't been selected, show error state "Please select all your options above". See designs.

Implementation details:
Error message "Please select all your options above" is in the same place as "Out of Stock" error

This is a similar interaction with the forms during checkout
Working example on NTO demo: https://nto-pwakit-production.mobify-storefront.com/undefined/product/2010033?color=BE2

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