fix(alexa-sdk): added this.i interface for i18n

> Once you are done defining and enabling language strings, you can access these
> strings using the this.t() function. Strings will be rendered in the language
> that matches the locale of the incoming request.

Extract from [alexa skills kit sdk
readme](https://github.com/alexa/alexa-skills-kit-sdk-for-nodejs)

types/alexa-sdk/index.d.ts

@@ -39,6 +39,7 @@ interface Handler {
     context: any;
     name: any;
     isOverriden: any;
+    t: (token: String) => void;
 }
 
 interface Context {