Add types to exports

When using this library with typescript I get an error:

```
There are types at '.../node_modules/@bramus/specificity/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@bramus/specificity' library may need to update its package.json or typings.
```

I think the fix is just to add the `types` field under the exports as well