Export TransitionStatus from react-transition-group

You can currently not access TransitionStatus which means it's impossible to create functions that accept this type as arguments, for example:

```javascript
const getTransform = (status: TransitionStatus) => {
  switch (status) {
    case ENTERED:
      return {};
    case ENTERING:
      return {};
    default:
      return {};
  }
};
```

types/react-transition-group/Transition.d.ts

@@ -16,7 +16,7 @@ export interface TransitionActions {
     exit?: boolean;
 }
 
-type TransitionStatus =
+export type TransitionStatus =
     typeof ENTERING |
     typeof ENTERED |
     typeof EXITING |