Add opinionated code quality tools

[tobeycodes]

I think we should consider adding some more PHP tools for code quality

• PHP Unit - I recently setup a project and added 10up/wp_mock and Pest PHP.
• PHPMD - Similar to PHP Stan we can add a baseline to ignore all the existing errors
• Rector - Auto refactor code, this is really good to make sure you keep code up to date and make things like PHP upgrades seamless

All of these tools can be and should be opt in. If you don't run the commands or have a blocking CI job then they just stay silently unused in the repository. We can add documentation and/or commands to remove them for projects that definitely do not want them

[claytoncollie]

+1 for these tools

Linking to the existing PR to implement PHPStan --> #133

[tobeycodes]

@claytoncollie Thanks for sharing the link to that. I am using a modified version of that PR in an existing project and it works quite well.