Add `sideEffects: false` to package.json

[neoncube2]

Hello! :)

Can we please add sideEffects: false to package.json? This can enable more tree-shaking, in some cases: https://github.com/webpack/webpack/blob/main/examples/side-effects/README.md

Other popular libraries, such as axios, have already added this flag: axios/axios#5025

I'd be happy to open a PR :)