Add PHPCS, fix errors, and add GH Action for checking PHP

[kadamwhite]

This PR builds upon #46 and introduces the same PHPCS configuration used upstairs in the main Diff site repo.

• Introduce Diff PHPCS configuration based on WPCS v3
• Run phpcbf to auto-fix many issues (mostly converts long array()s to short [] arrays)
• Manually resolve remaining PHPCS issues
  • Fix a variety of docblock issues
  • Remove unused parameters from functions
  • Escape site tagline on output
  • Add a version to the customizer script
• Remove unnecessary autoloader and load credits class directly
• Remove pass-through fields file
• Add PHPCS GitHub Action

[kadamwhite]

PHPCS annotations are working!: