Add Unit tests CI, and Unit Tests and Code Sniff for PRs

[mattyrob]

Create three Workflows for PHPUnit testing on pushes to main branch, PHPUnit tests for PRs and Code Sniff checks for PRs

All Submissions:

• [X] Have you followed the Classic Commerce Contributing guideline?
• [X] Does your code follow the repo standards?
• [X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes #350. So far I have a matrix testing PHP 7.0 to 7.4. 8.0 can be added but will likely need to allow failure for the moment. There is more to do in both:

• removing the legacy Travis code
• identifying which upstream GH Actions are required for back-porting based on what functions they perform and whether the is required.

How to test the changes in this Pull Request:

1. The PR tests have been tested on my fork
2. Once committed the CI action should run on pushes to the main branch of the code

Screenshot - before:

N/A

Screenshot - after:

Other information: