added oswalkdir.nim for the JS target

lib/pure/oswalkdir.nim

@@ -0,0 +1,16 @@
+
+type
+  PathComponent* = enum   ## Enumeration specifying a path component.
+    pcFile,               ## path refers to a file
+    pcLinkToFile,         ## path refers to a symbolic link to a file
+    pcDir,                ## path refers to a directory
+    pcLinkToDir           ## path refers to a symbolic link to a directory
+
+proc staticWalkDir(dir: string; relative: bool): seq[
+                  tuple[kind: PathComponent, path: string]] =
+  discard
+
+iterator walkDir*(dir: string; relative=false): tuple[kind: PathComponent, path: string] {.
+  tags: [ReadDirEffect], compiletime.} =
+  for k, v in staticWalkDir(dir, relative)):
+    yield (k, v)