Use `application/typescript; charset=utf-8` content type for `.ts` files

Same as Deno:

```console
> httpstat "https://deno.land/std@0.111.0/http/server.ts"

HTTP/2 200 
date: Sun, 17 Oct 2021 07:04:11 GMT
content-type: application/typescript; charset=utf-8
content-length: 18560
…
````