Be more lenient with appending `PATHSEP` in `common.path_suggest` in Windows

This allows to use the Unix separator without resulting in ugly 
suggestions that added the Windows separator too.

For example:
Before: `data/` -> `data/\core\`
After: `data/` -> `data/core\`

data/core/common.lua

@@ -164,7 +164,9 @@ function common.path_suggest(text, root)
     end
   end
 
-  if path:sub(-1) ~= PATHSEP then
+  -- Only in Windows allow using both styles of PATHSEP
+  if (PATHSEP == "\\" and not string.match(path:sub(-1), "[\\/]")) or
+     (PATHSEP ~= "\\" and path:sub(-1) ~= PATHSEP) then
     path = path .. PATHSEP
   end
   local files = system.list_dir(path) or {}