Provide type definition for TypeScript

Not a bug, but a feature request: Many front end developers nowadays write their scripting code in TypeScript instead of JavaScript. It would be great if this project would provide type definitions for the data-layer object.

Even better would be, if it would be possible to just import it in any .ts file and use it directly as an object without relying on the window object. Because like this we can make sure the object is there as it will be in the same transpiled js file and we don't need the [] fallback.