Mismatch between types in `pretender.js` and in `index.d.ts`

[vitch]

I can see that the sourcecode here was converted to TypeScript in #317.

How is the index.d.ts file here generated? I'm asking because it seems like some types are out of sync with the actual code (e.g. ExtraRequestData has method and url in pretender.ts but not in index.d.ts).

I'm happy to add the props but I was wondering if generation of the file should it be part of the release process or if there should there be a script exposed in package.json rather than manually editing index.d.ts?

cc @rpemberton