feat: add new child theme, newspack-joan

[thomasguillot]

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:

This is still a WIP.

Since the Block Theme is delayed, I’m experimenting with a new child theme for our classic theme. It draws inspiration from the Block Theme and the Newspack UI, both in its visual style and in the way its layout and grid are structured.

How to test the changes in this Pull Request:

1. Switch to this branch
2. Activate the new theme
3. Test all the pages, settings, etc...

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?