enum ALIGNMENT needs exported to pass values to scrollToAlignment prop with TypeScript

Hello  🙂

I am using `react-tiny-virtual-list` in a real-world TypeScript project.

Apparently the type of scrollToAlignment prop has changed from `string` in v2.1.4 to `enum ALIGNMENT` in v2.2.0 and scrollToAlignment props which is not available at the moment in TypeScript because `enum ALIGNMENT` is not exported in `index.d.ts`

I could only use `scrollToAlignment` prop with a value of appropriate type namely `enum ALIGNMENT` as follows:

in some-project > node_modules/react-tiny-virtual-list/types/index.d.ts
```
export { DIRECTION as ScrollDirection, ALIGNMENT as scrollAlignment } from './constants';
```

in some-project > some-component
```

```

Appreciated if `scrollToAlignment` is available to use in TypeScript in v2.2.0.

I made the following PR to offer a solution to this issue:
`https://github.com/clauderic/react-tiny-virtual-list/pull/69`