add command to open user directory in OS file manager

[sauerbraten]

Using https://docs.microsoft.com/en-us/windows/win32/api/shlobj_core/nf-shlobj_core-shopenfolderandselectitems?redirectedfrom=MSDN on Windows, xdg-open (maybe as lib?) on Linux, https://stackoverflow.com/questions/35807251/how-do-i-programmatically-open-a-folder-in-the-finder-without-selecting-anything on macOS.

If a content file path is given as first argument, open the location (zip, package dir, user dir, installation dir) instead of the user dir.