Create a typescript declaration file for the viewer

We need to create [d.ts](https://www.google.com/search?sxsrf=ACYBGNSxcJIiyZut5UzSKbkm0S8VHMZd3w%3A1574272996370&ei=5H_VXbabFqKd5wKgkZTYCw&q=declaration+file+typescript&oq=declaration+file+typescript&gs_l=psy-ab.3..0i7i30j0j0i8i7i30l4j0i30l2j0i5i30l2.17039.17039..17395...0.3..0.78.78.1......0....1..gws-wiz.......0i71.Y5u3CCOEcks&ved=0ahUKEwi2nen1r_nlAhWizlkKHaAIBbsQ4dUDCAs&uact=5) file for the [viewer-interface](https://github.com/ampproject/amphtml/blob/master/src/service/viewer-interface.js) for consumption for [amp-email-viewer](https://github.com/ampproject/amp-email-viewer). By doing this we can ensure that any changes to the viewer will propagate to the amp-email-viewer which currently duplicates values defined in viewer related code/service as opposed to importing it. This will be useful moving forward for users who have an interest in creating a custom viewer and understanding the interface.

Ideally, we should extract out all viewer related info and constants to a separate repo e.g. ampproject/viewer and push them to the amproject/amphtml and ampproject/amp-email-viewer?

Alternatively (via @fstanis ) use https://github.com/google/copybara to copy viewer related info to amp-email-viewer

Start with src/service/viewer-interface.js and compare its viewer-related values with the duplicated code in amp-email-viewer. Define the TypeScript declaration for the viewer interface so amp-email-viewer and custom viewers can consume it; the issue also mentions evaluating a shared viewer repository or Copybara, so that scope needs clarification before implementation.

Written by the indexing model from the issue text.