Support block themes for add to cart button

[bekarice]

Adds a CSS class needed for block themes to appropriately style buttons.

Fixes #

Description

Without the wp-element-button class applied, the add to cart buttons within the Add to Cart with Options block doesn't adopt any of the expected styles. Adding this class, when using add to cart blocks, resolves the styling issue. I tested this with the Twenty Twenty-Four theme.

How to test this PR

• Use Twenty Twenty-Four
• Adjust your stylebook to set button styles / colors / etc
• Create the single-product template using blocks, ensuring that Add to Cart with Options is used
• Observe that the buttons are unstyled
• Apply this fix, and see they now adopt the theme styling

Product impact

• [ ] Added changelog entry (or does not apply)
• [x] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [ ] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [ ] Added deprecated functions, hooks or classes to the spreadsheet

[bekarice]

@mattallan thanks! ♥️ didn't see any other instances of buttons that would need this fix (maybe the switching buttons?), but would be great to double check!

[mattallan]

didn't see any other instances of buttons that would need this fix (maybe the switching buttons?), but would be great to double check!

@bekarice yeah I had a look too and couldn't see any other <button> uses across subscriptions core and the main plugin so we should be good.

Looks like we use <a href> styled as buttons a lot