fix(trash-win): don't hang when shellslash is enabled

[TheLeoP]

Fixes #590

For some reason beyond my understanding, at least one of the following
things requires noshellslash to avoid the embeded powershell process to
send only "" to the stdout (never calling the callback because
"===DONE(True)===" is never sent to stdout)

• vim.fn.jobstart
• cmd.exe
• powershell.exe

shellslash has been removed from Neovim nightly so I didn't put that much time into investigating why or how this is happening.