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.

PFERD/utils.py

@@ -91,8 +91,14 @@ def url_set_query_params(url: str, params: Dict[str, str]) -> str:
     return result
 
 
+def str_path(path: PurePath) -> str:
+    if not path.parts:
+        return "."
+    return "/".join(path.parts)
+
+
 def fmt_path(path: PurePath) -> str:
-    return repr(str(path))
+    return repr(str_path(path))
 
 
 def fmt_real_path(path: Path) -> str: