Added PHPStan for Static Analysis

[n7studios]

Summary

Adds PHPStan for static analysis, covering:

• Correct variable declarations
• More accurate typehints in docblocks when specifying or returning arrays
• Moving InvalidArgumentException checks onto individual if blocks to prevent false positive reports in PHPStan
• Passing valid variable types to functions such as preg_replace
• Improved type checking on return values such as json_encode

Testing

Existing tests pass

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)