Export Typescript definition for what a Spec is

For example, this definition is one that I have written to infer it externally, but it would be more convenient to have an official spec from the chart library itself.

```typescript
type InferPropType<T> = T extends React.FunctionComponent<infer P> ? P : T;
type SeriesSpec = InferPropType<typeof LineSeries> &
  InferPropType<typeof BarSeries> &
  InferPropType<typeof AreaSeries>;
```