always use POSIX path sep / even under Windows

reason: consistency; passing test

Author: summivox

src/command.ls

@@ -202,8 +202,8 @@ switch
   filename = output-filename source, json
   dir = path.dirname source
   if o.output
-      dir = path.join that, dir.slice if base is '.' then 0 else base.length
-  js-path = path.join dir, filename
+      dir = path.posix.join that, dir.slice if base is '.' then 0 else base.length
+  js-path = path.posix.join dir, filename
   !function compile
       e <-! fs.write-file js-path, js.to-string! || '\n'
       return warn e if e