Add check for functions ending in __premium_only - #1447

What?

Closes #1443

Adds a static check Premium_Only_Function_Check in the Plugin Repo category to detect PHP function declarations whose names end with the __premium_only suffix.

As a potential future enhancement, this check could be upgraded to a token-aware sniff to ensure occurrences of __premium_only inside PHP comments, docblocks, or string literals are completely ignored.

Why?

WordPress.org plugin directory guidelines do not allow code intended strictly for a paid/pro/premium version to be included in free plugin submissions. Detecting common naming conventions like __premium_only helps authors identify and relocate premium-only code before submitting plugins for review.

How?

• Created class extending Abstract_File_Check.
• Uses regex pattern to scan PHP files and report a premium_only_function_found error.
• Registered the function check in Default_Check_Repository.
• Added PHPUnit unit test suite and fixture test plugins.

Testing Instructions

1. Run PHPCS linting and static analysis:

composer lint
composer phpstan

2. Start test environment stack:

npm run wp-env:start:tests

3. Run the new unit test suite:

npm run test-php -- --filter=Premium_Only_Function_Check_Tests

4. Verify that both tests pass - 2 tests, 9 assertions.

AI Usage Disclosure

• This PR was created without the help of AI tools
• This PR includes AI-assisted code or content

If AI tools were used, please describe how they were used:

• AI Tool: Gemini CLI
• Usage: Used to generate boilerplate code for Premium_Only_Function_Check, draft PHPUnit unit test cases, and verify WordPress coding standards compliance. Evaluated architectural trade-offs and manually verified all code, tests, and static analysis outputs.