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.