Adding sideEffects: false to package.json

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### Feature Request

A while back, I asked to add `"sideEffects": false` to this package's `package.json` file, to enable better tree shaking: https://github.com/FormidableLabs/react-fast-compare/issues/136

Anyway, I've found another reason that it might make sense to add this flag, which is that without it, if this module is imported but unused, its code still gets bundled.

See my comment at https://github.com/eugenezinovyev/react-fontawesome-svg-icon/issues/36#issuecomment-3663844017 for a more detailed explanation.

I'd reopen https://github.com/FormidableLabs/react-fast-compare/issues/136, but it says I don't have permissions XD

Thanks :)