Rule proposal: sort export

I'd like to be able to sort exports in a file when file only contains exports, e.g.

```js
// @flow

export type ScraperSourceType = {
country: string,
nid: string,
name: string,
url: string
};

export type ScraperFeaturesType = {
hasTimeAttributes: Array,
hasTimeUrls: boolean
};

export type ScraperType = {
source: ScraperSourceType,
features: ScraperFeaturesType,
scrapeVenues: Function,
scrapeMovies: Function,
scrapeShowtimes: Function
};
```

This file only contains exports. I'd like the latter to become into:

```js
// @flow

export type ScraperFeaturesType = {
hasTimeAttributes: Array,
hasTimeUrls: boolean
};

export type ScraperSourceType = {
country: string,
nid: string,
name: string,
url: string
};

export type ScraperType = {
source: ScraperSourceType,
features: ScraperFeaturesType,
scrapeVenues: Function,
scrapeMovies: Function,
scrapeShowtimes: Function
};
```

The issue does not name implementation files or tests. Start by locating the plugin's existing sorting or export-related rules and their test fixtures, then determine how a rule should recognize files containing only exports. Done means the requested export ordering is enforced without changing files that contain other statements.

Written by the indexing model from the issue text.