feat(newspack-components): add divider component

[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 PR adds a new component: Divider. I added a readme to explain how to use this in the future.

How to test the changes in this Pull Request:

1. Switch to this branch
2. Check if Dashboard still works
3. Check Components Demo.

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?

[dkoo]

The 100vw width doesn't work when it's used inside a container-type element:

We could add an exception for when it's inside a newspack-card element with borders, but I'm not sure that would catch all instances like this. IMO full-width shouldn't be the default alignment.

Also, React recommends using function-style components over class-style components, so I refactored this to a function-style component in 37f3897.

[thomasguillot]

@dkoo PR updated with better defaults 98932a8

[github-actions]

Hey @thomasguillot, good job getting this PR merged! 🎉

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