js-on-sx: JS functions accept extra args silently

SX strictly arity-checks lambdas; JS allows passing more args than
declared (extras accessible via arguments). Was raising "f expects
1 args, got 2" whenever Array.from passed (value, index) to a
1-arg mapFn. Fixed in js-build-param-list: every JS param list
now ends with &rest __extra_args__ unless an explicit rest is
present, so extras are silently absorbed.
conformance.sh: 148/148.

lib/js/transpile.sx

@@ -930,7 +930,7 @@
   (fn
     (params)
     (cond
-      ((empty? params) (list))
+      ((empty? params) (list (js-sym "&rest") (js-sym "__extra_args__")))
       ((and (list? (first params)) (js-tag? (first params) "js-rest"))
         (list (js-sym "&rest") (js-sym (nth (first params) 1))))
       (else