[php-src] Issue #24703: Allow Closure interface

Issue: https://github.com/php/php-src/issues/24703 Author: mvorisek ### Description This is a feature request to add ClosureInterface interface. It would allow anonymous functions to be runtime checked againts interface using LSP. What I expect: 1. Closure class to always extend ClosureInterface. 2. interface like: ``` interface FooClosure extends ClosureInterface { public function call(object $newThis, bool $p); } ``` would accept function (bool $x) {} but not function (int $x) {}. 3. throw when class/enum tries to implement it: ``` class Foo implements ClosureInterface {} ``` related past RFCs: - https://wiki.php.net/rfc/typesafe-callable - https://wiki.php.net/rfc/callable-interfaces