Commit composer.lock to git

[unfulvio-godaddy]

Description of the Change

It's advantageous that composer.lock is commited to the repository. This introduces more consistency when dependencies are installed by individuals for projects depending on WP_Mock.

If someone wants to learn more about why this is important (if you have a better article, please post in comment!):

• https://www.amitmerchant.com/why-you-should-always-commit-the-composer-lock-file/
• https://www.codementor.io/@decodeweb/importance-of-composer-lock-in-git-14v9ujxn9z
• https://m.dotdev.co/why-the-composer-lock-file-matters-c8d1e1488be7

One notable update since #167 is maybe bumping PhpUnit to the ^9.5 constraint -- however the previous constraint >=7.0 made it so that 9.5.x could have been installed too.

I've also removed sebastian/comparator from dev dependencies. Not sure what this is used for since I couldn't find any reference to it. Maybe it was required in older phpunit versions?

As for behat and php-coveralls we might still use those later on.

Closes #153

How to test the Change

• Review composer.json and composer.lock changes
• Unit tests pass

Changelog Entry

• composer.lock is commited to git

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly. n/a
• I have added tests to cover my change. n/a
• All new and existing tests pass.

[github-actions]

[agibson-godaddy]

Confirmed everything looks compatible back to PHP 7.3. (Nothing requiring PHP 8.0+, for instance.)