Add typescript types

[styfle]

Fixes #51

[lizthegrey]

@toshok, would you mind taking a look? this is blocking me from creating an otel exporter.

src/honey.ts:23:27 - error TS7016: Could not find a declaration file for module 'libhoney'. '/home/lizf/hny/opentelemetry-exporter-js/node_modules/libhoney/dist/libhoney.cjs.js' implicitly has an 'any' type.
  Try `npm install @types/libhoney` if it exists or add a new declaration (.d.ts) file containing `declare module 'libhoney';`

23 import * as libhoney from 'libhoney';
                             ~~~~~~~~~~

[styfle]

FYI, these types were created a year ago for libhoney 1.2.1 so I have no idea if they are valid for 2.x 🤷‍♂