Add assignment js-type to (language javascript)

* module/language/javascript.scm
  (assign): new js-type
  (print-exp, unparse-js): Handle case.
* module/language/javascript/simplify.scm (flatten-blocks):  Handle case.

module/language/javascript/simplify.scm

@@ -15,6 +15,8 @@
     (fold-right flatten '() stmts))
   (define (flatten-exp exp)
     (match exp
+      (($ assign id exp)
+       (make-assign id (flatten-exp exp)))
       (($ const c) exp)
       (($ new exp)
        (make-new (flatten-exp exp)))