Starting work to add patterns

[kimcoleman]

All Submissions:

• Have you followed the Contributing guidelines?
• 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:

This PR introduces 15 patterns as the building blocks for our forthcoming full page layouts.

These patterns can be added to a page or post via the block inserter. You can also browse all available patterns in the Appearance > Design > Patterns screen.

From that screen, a single pattern can be duplicated to a local wp_block and kept synced or un-synced so sites can have their own reusable versions of these layouts.

All patterns should respect the color scheme of of the site and look great with a dark background or light background color.

Other information:

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