js-on-sx: parser handles comma operator (a, b, c)

Was failing with "Expected punct ')' got punct ','" because the
paren handler only consumed a single assignment. Added
jp-parse-comma-seq helpers that build a js-comma AST node with
the expression list; transpiler emits (begin ...) so each is
evaluated in order and the last value is returned.
built-ins/Object: 44/50 → 46/50. conformance.sh: 148/148.

lib/js/parser.sx

@@ -418,17 +418,43 @@
               (dict-set! st :idx saved)
               (jp-advance! st)
               (let
-                ((e (jp-parse-assignment st)))
+                ((e (jp-parse-comma-seq st)))
                 (jp-expect! st "punct" ")")
                 e)))
           (do
             (dict-set! st :idx saved)
             (jp-advance! st)
             (let
-              ((e (jp-parse-assignment st)))
+              ((e (jp-parse-comma-seq st)))
               (jp-expect! st "punct" ")")
               e)))))))
 
+(define
+  jp-parse-comma-seq
+  (fn
+    (st)
+    (let
+      ((first-expr (jp-parse-assignment st)))
+      (if
+        (jp-at? st "punct" ",")
+        (jp-parse-comma-seq-rest st (list first-expr))
+        first-expr))))
+
+(define
+  jp-parse-comma-seq-rest
+  (fn
+    (st acc)
+    (do
+      (jp-advance! st)
+      (let
+        ((next-expr (jp-parse-assignment st)))
+        (let
+          ((acc2 (append acc (list next-expr))))
+          (if
+            (jp-at? st "punct" ",")
+            (jp-parse-comma-seq-rest st acc2)
+            (cons (quote js-comma) (list acc2))))))))
+
 (define
   jp-collect-params
   (fn