js-on-sx: rest in array pattern + nested pattern tolerance

Array destructure now supports [a, ...rest]. Rest entry is
transpiled to (define name (js-list-slice tmp i (len tmp))).

Nested patterns like [[a,b], c] now parse (as holes) instead of
erroring. jp-skip-balanced skips nested groups.

440/442 unit (+2), 148/148 slice unchanged.

lib/js/transpile.sx

@@ -951,6 +951,17 @@
       ((empty? names) tail)
       ((= (first names) nil)
         (js-vardecl-arr-forms (rest names) tmp-sym (+ i 1) tail))
+      ((and (list? (first names)) (= (first (first names)) "rest"))
+        (cons
+          (list
+            (js-sym "define")
+            (js-sym (nth (first names) 1))
+            (list
+              (js-sym "js-list-slice")
+              tmp-sym
+              i
+              (list (js-sym "len") tmp-sym)))
+          tail))
       (else
         (cons
           (list