Add "realpath" support for custom stream wrapper

[mvorisek]

Description

Currently, applications relying on realpath cannot be virtualized using VFS as realpath is supported for real file pathes only.

This is a feature request to allow realpath to be implemented by a custom stream wrapper.

Preferably explicitly (if a method is implemented) and implicitly (if a method is not implemented) assuming one VFS scheme represents one file system.

[mvorisek]

Please fix this, a lot of code relies on realpath to return a string - today, I experienced the following issue:

https://github.com/atk4/core/blob/5.2.0/src/ExceptionRenderer/RendererAbstract.php#L238 was returning false because the class path was comming from phar (phar://C:/wamp/www/vzd/laila/vendor/phpstan/phpstan/phpstan.phar/vendor/composer/ClassLoader.php).