Added TextEncoder and TextDecoder to window

[Mischala]

No description provided.

[msftclas]

All CLA requirements met.

[sandersn]

Shouldn't these types from inputfiles/lib/Encoding.widl already? Why do these need to be manually added to addedTypes.json?

[Constructor(optional DOMString label = "utf-8", optional TextDecoderOptions options),
 Exposed=(Window,Worker)]
interface TextDecoder {
  USVString decode(optional BufferSource input, optional TextDecodeOptions options);
};
TextDecoder includes TextDecoderCommon;

[Mischala]

The types exist, yes.
However, they are not applied to Window.

As I said in the PR description, calling new TextEncoder() works, however, new window.TextEncoder() does not.
This is because they type exists in scope, but has not been applied to the Window object.