refactor: product metabox for fields attach

[Soare-Robert-Daniel]

Summary

Made the interface look like the one from PRO for PPOM in the product meta box.

Will affect visual aspect of the product

YES

Screenshots

Test instructions

• Go to WooCommerce > All Products and choose a product.
• On the product page, check the metabox tab PPOM Fields
• You should see the Upsell, and all the functionality should work.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/ppom-pro/issues/395

[pirate-bot]

Plugin build for 9be5ec9 is ready 🛎️!

• Download PPOM - build 🔗

[HardeepAsrani]

Code looks good, you can fix the textdomain issues and it's good to go.

[pirate-bot]

🎉 This PR is included in version 33.0.0 🎉

The release is available on GitHub release

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