Add sideEffects: false to enable tree shaking

[anajavi]

This PR enables webpack 4 to do tree shaking on es-build.

It adds sideEffects: false to package.json, which tells webpack that imports don't have sideEffects. https://webpack.js.org/guides/tree-shaking/

[codecov]

Codecov Report

Merging #295 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #295   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          14       14           
  Lines         110      110           
  Branches       36       36           
=======================================
  Hits          105      105           
  Misses          4        4           
  Partials        1        1

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[timche]

Cool, thanks 👍