Library is not tree-shakeable

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This library does not have a "module" entry in its package.json pointing to an ESM build, and it doesn't have a "sideEffects" declaration in its package.json, so it's not possible for Webpack to do any tree-shaking optimizations on it. The end result in my app is that a single import like this one pulls the whole library:

import { routerMiddleware } from 'connected-react-router';

You can see react-router properly configured with these two lines: https://github.com/ReactTraining/react-router/blob/447394fbf79116c7d7283ababa90b0289fb0290f/packages/react-router/package.json#L30-L31

@supasate would you be open to merging a PR that fixes this?