io.files.trash.windows: convert input path to absolute in send-to-trash

io.files.trash.windows: convert input path to absolute in send-to-trash

The input path must be absolute, but normalize-path can't be used, because
that returns UNC path, and SHFileOperation fails on any path prefixed with
"\\?", see https://docs.microsoft.com/en-us/windows/win32/api/shellapi/ns-shellapi-shfileopstructa
Use absolute-path instead. The mixture of slashes and backslashes in the
path is tolerated, at least on Windows 10.

Add a simple unit-test.