Add config.xsd

[kelunik]

It'd be useful to still be able to use the correct version of the config.xsd when using the phar.

[zonuexe]

psalm.xml has vendor/vimeo/psalm/config.xsd hardcoded, so it would be useful to be able to replace it with vendor/psalm/phar/config.xsd.

    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"

refs laminas/laminas-ci-matrix-action#301