Always use '/' as path separator for regex rules

Previously, regex-matching paths on windows would, in some cases, require four
backslashes ('\\\\') to escape a single path separator. That's just too much.

With this commit, regex transforms now use '/' instead of '\' as path separator,
meaning rules can more easily be shared between platforms (although they are not
guaranteed to be 100% compatible since on Windows, '\' is still recognized as a
path separator).

To make rules more intuitive to write, local relative paths are now also printed
with '/' as path separator on Windows. Since Windows also accepts '/' as path
separator, this change doesn't really affect other rules that parse their sides
as paths.

CHANGELOG.md

@@ -22,6 +22,9 @@ ambiguous situations.
 
 ## Unreleased
 
+### Changed
+- Use `/` instead of `\` as path separator for (regex) rules on Windows
+
 ## 3.0.1 - 2021-06-01
 
 ### Added