[internal] add sideEffects: false to pkg for webpack 4

[hshoff]

🏠 Internal

• [internal] add sideEffects: false to pkg for webpack 4

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In preparation for webpack 4, this change will tell webpack to it's safe to prune unused exports.

This benefits developers using webpack for perf gains in the form of smaller bundle sizes.

Until webpack 4 is out, it's preferred to import from the deep path for vx users (only include what you use in your build vs including the entire lib index).

import AxisBottom from '@vx/axis/build/axis/AxisBottom';

more info:

• d3 discussion: Add sideEffects: false to package.json.  d3/d3#3131
• https://github.com/webpack/webpack/tree/next/examples/side-effects
• related discussion: Add module support for tree shaking #143