Support additional ways to mark parameters as unused that other tools can use?

[TysonAndre]

With PHP 8.0 unexpectedly adding named argument support, renaming parameters to indicate that they're unused turned out to be a mistake for me to suggest (months before learning about that).

            'possiblyUnusedParamWithUnusedPrefix' => [
                '<?php
                    class A {
                        public static function foo(int $unusedArg = null) : void {}
                    }

                    A::foo();',
            ],

Instead, it would be useful to add @unused-param as a modifier - see phan/ProposedAnnotations#1
(e.g. with syntax such as the following)

/**
 * @param $x @unused-param
 * @param int $y
 * @unused-param $y
 * @unused-param ...$args
 */
function example(int $x, int $y, ...$args) {}