Add operating system type to has() function

[chylex]

I tried to replicate the logic from https://github.com/vim/vim/blob/0f058d13206665bad37c7d42834cfa0075f50239/src/evalfunc.c#L5511, not 100% sure if I got it right (and some special cases like win32unix are missing) but hopefully it's ok.

[AlexPl292]

I see there is a pretty hairy set of ifs there, but I think we can have a simplified version. Thank you!

[chylex]

Yea, wasn't sure if there's a better way to write it considering all the quirks of how vim decides which OS features to use, like win32 being included in 64-bit Windows, or how MacOS is based on BSD but isn't supposed to have the bsd feature. I initially considered a switch over OS.CURRENT, but that doesn't include all the options (Solaris).