Export sass vars to css var

[kevinruscoe]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' 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:

Simply exports the SASS variables in _variables as CSS variables, so aid with front-end development.

Closes #28707.

How to test the changes in this Pull Request:

1. Add h1 {color: var(--wc-green) !important;} to your stylesheet to see the h1 header be the WooComms green colour.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?

Changelog entry

Enhancement - Export SASS variables as CSS variables.

[claudiosanches]

Thanks for your help!