perf: enable tree shaking for Webpack

[levithomason]

This PR sets sideEffects: false in our package.json to inform Webpack that our library is safe for tree shaking.

https://webpack.js.org/guides/tree-shaking

[dvdzkwsk]

[brianespinosa]

@levithomason how much did you benefit in package size from this change? I see you are using Webpack 3. I didn't realize sideEffects: false also worked for this version. The current Webpack docs are for Webpack 4.

[levithomason]

The docs will always import 100% of the features so the size will be maxed at all times. The change here was made for consumers to benefit from.

We are currently working on library level perf tracking #141 and library level bundle size tracking #97. It would be nice to track partial use bundle size as well.

[levithomason]

@brianespinosa ^