Mark library as side-effect free for TreeShaking improvements

[intellix]

Webpack 4 added the ability to mark modules as being side-effect free. I'm not 100% sure but if this library isn't, we can mark individual files as having them. This was recently added to graphql-js as well :graphql/graphql-js#1312

Webpack docs: https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free