Use stderr for outputing warnings instead of stdout

[ostrolucky]

This fixes phpstorm integration, which cannot currrently correctly detect behat version because of output that's mixed with the output from this extension.

I even created jetbrains issue but it was difficult to properly formulate the issue at the time https://youtrack.jetbrains.com/issue/WI-76429

In the end doing the fix here would be much easier. I've tested this and it fixes the integration.

[dvdoug]

Hi @ostrolucky

Thanks for the PR. I've also tested this and it seems to work, but before I merge/release I'd like to understand why it works/what's the theory behind the change? The getErrorOutput() method doesn't appear to have any documentation on the Symfony side (the only reference I can see in the docs is in the Process component?). It's a secret "getter" on the output stream that switches things from stdout to stderr?

[ostrolucky]

Yes getErrorOutput is just a getter for output that directs to stderr

It's briefly mentioned here https://symfony.com/doc/current/components/process.html#usage, but uses long terminology ("standard output" instead of "stdout", "error output" instead of "stderr")