Add sideEffects flag to react-is for tree shaking (#27701)

## Summary This PR: - Adds the `"sideEffects": false` flag to the `react-is` package, to enable proper tree-shaking ## How did you test this change? React-Redux v9 beta switches its artifacts from separate JS files to pre-bundled artifacts. While testing builds locally, I noticed that our use of `react-is` was no longer getting tree-shaken from Vite builds, despite `react-is` only being used by our `connect` API and `connect` itself getting shaken out of the final bundle. Hand-adding `"sideEffects": false` to the `react-is` package locally convinced Vite (+Rollup) to properly tree-shake `react-is` out of the bundle when it wasn't being used. I'd love to see this published as v18.2.1 as soon as this PR is merged - we're hoping to release React-Redux v9 in the next few weeks!