WSL support improvements
WSL support improvements

- Implement `__done_get_focused_window_id` on WSL as a PowerShell
  script.
- Replace `command "$powershell_exe"` with `eval` because a variable
  cannot be used as a command path.
- Use the `-Silent` option of BurntToast when `__done_notify_sound` is
  set to zero.

fisherman/done