[JS] Ref #15952 make toOpenArray works better (#17001)

* ref 15952 toOpenArray works in JS

* fix

tests/openarray/topenarray.nim

@@ -0,0 +1,13 @@
+discard """
+  targets: "c cpp js"
+"""
+
+proc pro[T](a: var openArray[T]) = discard
+
+proc main =
+  var a = [1,2,3,4,5]
+
+  pro(toOpenArray(a, 1, 3))
+  pro(a.toOpenArray(1,3))
+
+main()