Fixes #8483 Windows completion error

This should fix #8483 I am not a Julia power user yet so could some of you guy's check it's working because I have not compiled base with it, @lucasb-eyer, @tkelman?
Since 47d3ecf the auto completion is completing with a ``\` in `shell>` and if `\\` where used then the auto completion did not work. This pull changes it to always auto complete and use `\\` in paths and fixes the completion behavior. As @tkelman mention this part could be skipped on windows. Else we should role `Base.REPLCompletions.shell_completions` back to before 47d3ecf and use `/` as path separator again in shell mode on windows.