[9.x] Makes with generic

[nunomaduro]

This pull request makes the with helper generic. The current type definition is not perfect, assuming that the return value from with callback will be same as the given value. Yet, it's better then what we had before. At PHPStan level 5 works perfectly, and PHPStorm, of course, is able to infer the types correctly.

[sebdesign]

@nunomaduro If think we should handle the case where the callback returns a different type than the value:

E.g.: https://github.com/laravel/framework/blob/master/src/Illuminate/Log/LogManager.php#L132
E.g.: https://github.com/laravel/framework/blob/master/src/Illuminate/Bus/Batch.php#L171

/**
 * Return the given value, optionally passed through the given callback.
 *
 * @template TValue
 * @template TReturn
 *
 * @param  TValue  $value
 * @param  (callable(TValue): TReturn)|null  $callback
 * @return TValue|TReturn
 */
function with($value, callable $callback = null)
{
    return is_null($callback) ? $value : $callback($value);
}

[sebdesign]

Your tests are asserting that the $user parameter is int|User, while it's User. The return type is int|User which is correct.

[nunomaduro]

@sebdesign I am aware of that. Yet, PHPStan does not support default generics values - meaning that the code you proposed will make a phpstan error. So this pull request does the possible job for now.

[sebdesign]

@nunomaduro thank you for your explanation! Indeed you are right we cannot use default/optional generics, I'm sorry for the noise on this PR.