Fix google-product-category-fields-loads.js loading on every page.

[rawdreeg]

Changes proposed in this Pull Request:

Closes #1949 .

admin/google-product-category-fields.js currently loads on all admin pages. This PR restricts it to the product_cat and edit-product_cat screens where it's needed for the google product category fields.

Issue #1949 makes other recommendations such as adding an async/defer attribute and moving the script to the footer, which I decided against because it would mean delaying the init execution which impacts UX:

https://github.com/woocommerce/facebook-for-woocommerce/blob/5156a45c02672057476573f0b79c58084f20f10f/includes/Admin/Google_Product_Category_Field.php#L30-L38

I also decided not to make changes to infobanner assets as we still need to evaluate whether this is currently needed. See #1981

• [ x] Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical.

Detailed test instructions:

1. Check out this branch.
2. Navigate to the admin dashboard, confirm google-product-category-fields-loads.js does not load on all page
3. Navigate to Admin -> Product -> Categories, confirm the google fields work as expected

Changelog entry

Fix - Ensure google-product-category-fields-loads.js loads only on the product category screens.

[message-dimke]

Hey, @rawdreeg !

Thanks for the PR, I can confirm google-product-category-fields.js loads at every Admin page before the code changes, but I can also confirm few more places than just Product > Category which require the file.

There are maybe more, these are two were on my memory.

[rawdreeg]

Thanks for catching this, @message-dimke. I have ensured the google-product-category-fields.js loads everywhere it's needed.

This is ready for another round of reviews :-)

[message-dimke]

LGTM! Thanks, @rawdreeg!