Adding `sideEffects: false` to package.json

[neoncube2]

Would it make sense to add sideEffects: false to package.json in the react,react-dom, use-sync-external-store, scheduler, etc. packages? It was already added to the react-is package: #27701

Adding sideEffects: false can enable more tree-shaking, in some cases: documentation.

[ZunedKhan07]

Hi, I’d like to help with this. I can go through the mentioned packages and add "sideEffects": false in the appropriate package.json files while ensuring no unintended side effects occur. Please let me know if there's any specific guideline or test I should run after making these changes.