Add GitHub Actions for Testing This Repo

[kevinfodness]

User Story

As a maintainer of this repo, I want to ensure that changes to this repo don't break automated tests when a full installation, including a plugin and theme, are performed by a user.

Acceptance Criteria

• Create a GitHub Action to ensure that changes to create-wordpress-plugin and create-wordpress-theme do not break this project.
• Ensure these checks run on PRs against this repo and on a schedule.
• Use the automatic installation script established in Add Installer Script #7 to ensure that the project is properly configured before testing, emulating user behavior.
• Test against a VIP-type repo and a Pantheon-type repo to ensure both targets are supported.