Add Typed Racket example to RacketScript playground?

Been thinking about TypeScript.

Is the Typed Racket example possible? 

```
#lang typed/racket

;; Using higher-order occurrence typing
(define-type SrN (U String Number))
(: tog ((Listof SrN) -> String))
(define (tog l)
  (apply string-append
         (filter string? l)))
(tog (list 5 "hello "
           1/2 "world" (sqrt -1)))
```
Cc: @samth