phpstorm.meta.php support for dynamic return types

[DCoderLT]

This PR starts to add support for PHPStorm’s metadata files. It only supports a few basic directives for resolving method/function return types, but of course support for other directives can be added.

I do not think this is ready for a merge yet, I would like some feedback before I dig deeper into it. For example:

• which other meta directives need to be supported (registerArgumentsSet)?
• what tests are needed?
• PHPStorm scans the entire project looking for these meta files, including vendor directories. Should PHPStan also look for vendored meta files by default? I’m a bit concerned about parsing time for large projects.