Improve TS type for onResultsChange callback parameter

The `onResultsChange` callback in `src/index.d.ts` has a loosely typed parameter:

```ts
onResultsChange: ({ type: string }) => void;
```

This should be a union type matching the MeasurementConfig types to provide better type safety:
```ts
onResultsChange: ({ type: 'download' | 'upload' | 'latency' | 'packetLoss' }) => void;
```
or
```ts
onResultsChange: (change: { type: MeasurementConfig['type'] }) => void;
```