Add codecov to track code coverage

[aaronjorbin]

Fixes #195.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: aaronjorbin <jorbin@git.wordpress.org>
Co-authored-by: JasonTheAdams <jason_the_adams@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aaronjorbin]

@JasonTheAdams @felixarntz It looks like there are alot of @covers that are not valid, either because they don't exist or they are calling code that listed as being covered. Was there a large refactor that removed classes and didn't update the tests? .

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[JasonTheAdams]

Thanks, @aaronjorbin! Looks like we currently have 87.38% code coverage!