[php-src] Issue #20728: `never` union type

Issue: https://github.com/php/php-src/issues/20728
Author: kkmuffme

### Description

```php
function foo(): bool|never {
    if (rand(0,1)) {
        echo "false";
        return false;
    }
    
    echo "exit";
    exit;
}

foo();
```

https://www.php.net/manual/en/language.types.never.php
>[never](https://www.php.net/manual/en/language.types.never.php) is a return-only type indicating the function does not terminate
>it cannot be part of a [union type](https://www.php.net/manual/en/language.types.type-system.php#language.types.type-system.composite.union) declaration

What if a function only terminates conditionally like in the example above? Currently using "bool" type hint works above, however that loses relevant information, which currently can only be conveyed with a phpdoc comment.

The docs state:
>**Therefore**, it cannot be part of a union type

but there is nothing "therefore" about it and no reason why it cannot be part of a union type
(except PHP itself "enforcing" no union types of overlapping built-in types - but if we look at the int/float behavior of allowing int, this isn't strictly true anyway, and not true at all when taking non-built in types into account, e.g. with child/parent classes)

This has been working in static analysis (psalm, phpstorm) since ages too and I think is somthing that should be natively supported too.
(I'm the one who implemented this in psalm back then)