feat(homepage-posts): add a show-full-content attribute

[adekbadek]

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:

Adds a "show full content" attribute on the Hompage Posts Block.

How to test the changes in this Pull Request:

1. Insert a HPB on a page
2. Observe there's a new setting in "Post Control Settings" sidebar panel
3. Toggle on "Show Full Content" and observe full post content is displayed
4. Publish the page, observe the full post content is displayed
5. Toggle it off, observe it's mutually exclusive with the "Show Excerpt" attribute

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?

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[matticbot]

🎉 This PR is included in version 4.2.0-alpha.4 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 4.2.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀