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/transpile.sx

@@ -165,6 +165,8 @@
             (js-transpile-new (nth ast 1) (nth ast 2)))
           ((js-tag? ast "js-class")
             (js-transpile-class (nth ast 1) (nth ast 2) (nth ast 3)))
+          ((js-tag? ast "js-comma")
+            (cons (js-sym "begin") (map js-transpile (nth ast 1))))
           ((js-tag? ast "js-throw") (js-transpile-throw (nth ast 1)))
           ((js-tag? ast "js-try")
             (js-transpile-try (nth ast 1) (nth ast 2) (nth ast 3)))