Unable to require graphqlUploadExpress module after the major bump to v15
Hi there, 

we import this module in NodeJS using commonJS `require` method:
```js
const { graphqlUploadExpress } = require('graphql-upload');
```

But after the major bump to v15 we started getting error:
```
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: No "exports" main defined in /Users/abc/dev/project/node_modules/graphql-upload/package.json
```

Which is thrown because there was removed `main` field in package.json.

Is there any other way how the `graphqlUploadExpress` should be imported now? 

Also, thanks for spending your time and working on this module ;-)