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!