Keep the "Add to cart" and show the message separately (not replacing) upon ajax call [#3557070]

Problem/Motivation

Before the vanilla js patch to remove jQuery dependencies, the "Add to cart" button behavior upon successful call would be showing a message after the button.
The new patch replaced the button entirely, which the first impression is that the product only can be add to cart once.
But upon refresh of the page, the button re-appear.

Steps to reproduce

Install the module > enable a content type > go to content > click "Add to cart"

Proposed resolution

Keep the "Add to cart" and show the message separately (not replacing) upon ajax call.

Issue fork basic_cart-3557070

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3557070-uiux-for-add changes, plain diff MR !25
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