Add a PHPStan configuration and run it as a GitHub Action

[dkotter]

Description of the Change

This PR adds in PHPStan as a dev dependency, along with the configuration to run that and a GitHub Action Workflow that will run on all PRs. I've also fixed up all issues that have been reported.

Right now I have this set to level 1 (lowest is 0, highest is 9). If I change this to level 2, there are 85 new things that get flagged (though most look to be pretty similar so solving a few should solve most of them). Ideally we bump this up to a higher level but I think starting at 1 is better than nothing.

How to test the Change

Verify the PHPStan Action runs and passes on this PR

Changelog Entry

Fixed - Addressed all issues raised by running PHPStan
Developer - Added PHPStan into our workflow with the ability to run that locally and in a GitHub Action

Credits

Props @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[iamdharmesh]

Thanks for adding this @dkotter. LGTM.