js-on-sx: js-to-string emits comma-joined elements for SX lists

String([1,2,3]) was returning "(1 2 3)" (the SX (str v) fallback in
js-to-string fell through for SX lists). Replaced the fallback with
a list-typed branch that delegates to (js-list-join v ","). Fixes
String(arr), "" + arr, and any implicit array-to-string coercion.
built-ins/String: 65/99 → 67/99. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1231,7 +1231,7 @@
                           "[object Object]"))
                       (js-to-string result)))
                   "[object Object]"))))
-          (str v))))))
+          (if (= (type-of v) "list") (js-list-join v ",") (str v)))))))
 
 (define
   js-template-concat