Support spawning child process on Windows without process I/O pipes

[clue]

This PR adds limited Windows support by omitting any process I/O pipes on Windows. While this means that portability might be limited, it at least allows spawning processes on Windows. Also, this PR includes plenty of tests and documentation for known workarounds, so one can in fact process output using I/O redirections. This has been tested extensively on some Windows 10 installations, but this repository does not currently run any tests on Windows. I consider this to be out of scope for this ticket, but agree that it makes sense to look into test automation in a follow-up PR. In the meantime, you're invited to run this on your local Windows installation or take my word for granted, so now let's get this shipped! ❤️

Resolves / closes#9
Builds on top of #65

[WyriHaximus]

[clue]

Updated to address the above remarks. Thanks for the review!

[WyriHaximus]

LGTM 🚢 🇮🇹 !

[ghost]

Sidenote: I/O pipes could be supported if PHP would implement support for named pipes (as opposed to anonymous pipes). If that support is expressed through being able to create and remove named pipes in userland or through some sort of internal creation of named pipes with a random name instead of anonymous pipes, is open.

[clue]

@CharlotteDunois I agree and I'm looking forward to see more alternatives in PHP userspace in the future! 👍 Some of those options have been discussed in #9 already, so for example we might also be able to set SO_OPENTYPE to SO_SYNCHRONOUS_NONALERT on Windows or other workarounds. I consider this PR to be a starting point and I would love to see more people contribute possible workarounds in the future!

[ghost]

🎉

[WyriHaximus]

🎉