normalise should eliminate parent directories on Windows

Windows seems to interpret paths differently than on POSIX systems.

- On POSIX, `/sym/link/../foo` refers to `/target/dir/../foo ≡ /target/foo` (`..` expands after dereferencing).
- On Windows, `C:\sym\link\..\foo` always refers to `C:\sym\foo` (`..` expands before dereferencing).

This feature would be useful to have because if you want to pass in extended-length paths (the `\\?\` prefix) you have to normalize the path before adding the prefix, otherwise the Windows API will interpret `..` literally.

Start by locating the normalise implementation and its existing tests in the filepath library. Compare current handling of parent directories on POSIX and Windows, then add coverage showing the requested Windows behavior and verify that extended-length paths can be normalized before prefixing.

Written by the indexing model from the issue text.