[Product form] Update the disabled attribute approach to work with no-js

[ludoboludo]

PR Summary:

Looking into a different approach in which we remove the disabled attribute faster without compromising no-js.

Why are these changes introduced?

Related to this PR that recently shipped

What approach did you take?

Remove the attribute via inline JS so it's doing it faster than waiting for product-form.js to load.

Other considerations

Different position to place the JS / Remove the repetitive removal of the disabled attribute in the product-form custom element JS.

Visual impact on existing themes

Testing steps/scenarios

• Go to the store preview,
• Open the inspect tool and go to the network tab, throttle the connection to a slow 3G one
• Reload the home page and try to click on Add to cart as soon as you see the button show up for the feat. collection section

Demo links

• Store
• Editor

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility