TextEncoder/TextDecoder not yet available

First of all, thank you for the great work behind DNT and Deno itself!

`TextDecoder` and `TextEncoder` cannot be used yet to compile to NPM modules. I know it is expected, just adding for reference.

Source:

```ts
const hexDecode = (s: string) => new TextEncoder().encode(s);

console.log(hexDecode("1234"));
```

Build output:

```
[dnt] Building project...
[dnt] Type checking...
src/mod.ts:9:38 - error TS2304: Cannot find name 'TextEncoder'.

9 const hexDecode = (s: string) => new TextEncoder().encode(s);
~~~~~~~~~~~

error: Uncaught (in promise) Error: Had 1 diagnostics.
throw new Error(`Had ${diagnostics.length} diagnostics.`);
^
at build (https://deno.land/x/dnt@0.27.0/mod.ts:299:15)
```

Thank you!