Upgrade tooling for WPCS 3.0 and PHP 8.0+

[kadamwhite]

• Update to WPCS 3.0, and migrate to a temporary standard adapted from HM's outdated standard but adjusted for modern PHP compatibility
• Pin versions of remaining Action dependencies missed in Security enhancements: specific commit to 3rd-party GH Actions, secure href attr on 404.html file #56
• Introduce PHPCS testing actions

[tomjn]

I understand pinning to versions but why pin to a specific commit of the actions?

[tomjn]

do we still need Travis if we have a php linting action?

[kadamwhite]

Unit tests! 😁

[tomjn]

Approved in principle from a reading of the code, left some comments

[jrfnl]

Thank you for your support and your enthousiasm embracing the take-over of the PHP_CodeSniffer package.

In contrast to earlier information, arrangements have been made to allow the package to continue under its original name on Packagist. The commit (in the new repo) to rename the package has been reverted.

I'd recommend reverting the Composer reference changes and keeping the changes which refer to the repo URL on GitHub.

Sorry for the confusion and thank you for understanding. I hope you'll enjoy the 3.8.0 release, which was released last month.