chore: get php linting working and fix errors

[laurelfulford]

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 gets PHP linting working, and lines up the linting settings closer to Newspack Blocks (including switching the linting from WordPress-Core -- left over from the theme's original forking of Twenty Nineteen -- to just WordPress). It also fixes any errors that came out of running composer lint.

There are still a number of exclusions in the phpcs.xml.dist that are unique to the theme, but removing them and making the subsequent fixes turned into a lot of changes for one PR, so I think that might be best to handle after this is merged.

Closes # .

How to test the changes in this Pull Request:

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?