Use `/` as `pathSeparator` also on Windows

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.)

Start by locating the pathSeparator definition and reviewing how its platform-specific value is used. Compare the golden-test failures linked in the issue, assess the stated backwards incompatibility and migration options, and consider the required major version change before defining done.

Written by the indexing model from the issue text.