Add `"sideEffects": false` in `package.json` of `core-js-pure` package for proper Webpack tree-shaking

[roshecode]

If possible please add "sideEffects": false, in package.json of core-js-pure package similar as it was done for lodash in this PR to let Webpack to tree-shake code. Webpack by default treats core-js-pure as it has side effects, but in reality it's side effect free ("pure"), so adding this flag will instruct Webpack to remove core-js-pure imports (in case they are not used further in code because of some conditions) leading to a smaller bundle size.

[zloirock]

Hmm, thanks, it could be a great addition. However, I'm not totally sure how it works and that it could be totally safe because of some specific cases. I'll explore it.

[zloirock]

It seems "sideEffects": ["./modules/*.js"] should work.

[roshecode]

It works, thank you!