Feature: adds a css class for the form design

[JasonTheAdams]

Description

This humble PR adds a class to the root div of a v3 form with the Form Design ID. Something like: givewp-donation-form-design--classic. It's purpose is so that when folks add custom styles (especially globally via code) they can use CSS to apply styles to only certain form designs.

Affects

V3 form rendering

Testing Instructions

1. Make a form
2. View the form on the front-end
3. Check the root div for the Form Design class — it should correspond to your selected Form Design
4. Switch Form Designs and check again

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Self Review of code and UX completed

[jonwaldstein]

Nice! ⭐