[#17] Fixed variable references in comments not being updated during …

…auto-fix. (#18)

[#15] Fixed false positives on variables prefixed with underscore.

Variables like `$_static_value` are now completely skipped from naming
convention checks as they are commonly used for internal/special variables.

[#13] Fixed nullable class properties incorrectly triggering `LocalVa…

…riableNaming.NotSnakeCase`.

Refactored case sniffs to allow choosing snake or camel case.

Enhance README with PHPUnit data provider rules

Added PHPUnit data provider naming conventions and organization rules.

Updated to be compatible with PHPCS4.

Allow installing in PHP 8.2

Simplify functional test assertions.

Separated rules into `LocalVariableSnakeCase` and `ParameterSnakeCase`.