Make ${file_path} return path with forward slashes on windows

[libbrfish]

Using sioyek on windows, I'm trying to write a new-command to open the current file in explorer:

new-command _open_in_explorer explorer /select,%{file_path}

does not work due to forward slashes which are not always supported in commands on windows (instead of backslashes).

If instead %{file_path} returns the path with backslashes, these issues on windows will be resolved.