add JsSyntax module
Rewrite JsSyntax module

* Completely rewrite pretty printing (currenttly does not currently
  handle the complete AST).
* Remove JsSyntax trait
* JsExpr should not extend JsStmt. Add implicit from JsExpr to JsStmt
  using new JExprS case class
* Add JsUnit case class
Update elm AST to convert to javascript AST and later into a string