feat: add a filter on term classes so other plugins can add classes

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A small feature which shouldn't affect any plugin behavior. Adds a filter for the term class names that are added to each post displayed in Homepage Posts and Post Carousel blocks. This will let other plugins hook into the filter and add arbitrary class names as needed. The specific use case I added this for is featured listings, but it might come in handy for other purposes as well.

How to test the changes in this Pull Request:

1. Check out this branch.
2. Somewhere in your theme's functions.php, temporarily add a callback for the new filter:

function newspack_test_term_classes_filter( $classes ) {
	$classes[] = 'my-arbitrary-class';
	return $classes;
}
add_filter( 'newspack_blocks_term_classes', 'newspack_test_term_classes_filter' );

3. Add a Homepage Posts and Post Carousel block to a post and view on the front-end. Confirm that your arbitrary class name is added to each article item in both blocks.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 1.38.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.38.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀