Add sniff for modern callback syntax

### Description

Rather than specifying action and filter callbacks as string representations of function names, or arrays of callables in class methods (e.g., `[ $this, 'my_callback_function' ]`), we should prefer modern (PHP 8+) callback syntax, e.g., `$this->my_callback_function( ... )` or `my_callback_function( ... )`.

### Use Case

When writing modern PHP, we should use modern callback syntax, since static code analysis tools and IDEs are better able to tie the specified function to its definition.

Start by locating the sniff that checks action and filter callbacks, then find its related tests or test entry point. Review the current handling of string callbacks and callable arrays, and define coverage for PHP 8+ first-class callable syntax; done means the sniff reports the older forms while accepting the modern forms described in the issue.

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