Add TypeScript typings

[kmxz]

Add TypeScript typings according to the specification in https://www.typescriptlang.org/docs/handbook/typings-for-npm-packages.html.

[hoxxep]

@fians In favour of adding this too 👍

[fians]

@kmxz is it typings already changed to types? Because I couldn't find the property on the page you share.

[kevin-brown]

@fians

Note that the "typings" field is synonymous with "types", and could be used as well.

So yes, it appears as though it was renamed to types.