Use `/` as `pathSeparator` also on Windows

[hasufell]

In GitLab by @andreasabel on Dec 8, 2022, 17:06

Windows supports / as path separator. (Apparently, since forever, which I didn't know, but this discussion even says DOS already accepted it: https://bytes.com/topic/python/answers/23123-when-did-windows-start-accepting-forward-slash-path-separator.)

Making pathSeparator uniformly '/' would increase the portability of Haskell CLI programs to Windows.
I think differentiating needlessly between OSs here is a hindrance to portability rather than an enabler of portability.

Here are examples where e.g. golden value testsuites break on Windows because of that non-uniformity of path separators:

• https://github.com/fugue/goldplate/issues/29
• https://github.com/agda/agda/issues/5549

Switching pathSeparator would of course be a major backwards-incompatibility, so there should be maybe a major-major version bump to 2.x. (If this issue is accepted, one could think about migration routes.)