Add typings to package.json.

[ivasilov]

According to this link the typings can be used directly from the NPM package, if specified in package.json. This PR adds support for it.

[spion]

Are you sure they can be ambient external modules? Last time I checked you'd have to leave the wrapper out, e.g. if the original had

declare module "anydb-sql" {
// definitions here
}

you would need to remove the declare wrapper:

// definitions here