More precise dirname() return type

[staabm]

given a non-empty-string-path, dirname() will return a non-empty-string

see e.g. https://3v4l.org/YK9T1#veol

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we applied the same logic already for pathinfo:

phpstan-src/src/Type/Php/PathinfoFunctionDynamicReturnTypeExtension.php

Line 50 in c86f75a

$builder->setOffsetValueType(new ConstantStringType('dirname'), new StringType(), !$pathType->isNonEmptyString()->yes());

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use case: prevents a

Configuration defined type for constant APP_ROOT (non-empty-string) does not accept value string.

error in define('APP_ROOT', dirname(__DIR__, 2)); when using dynamicConstantNames