Add "sideEffects: false" to package.json

[ericjeney]

This PR marks htmlparser2 as free of side effects to allow Webpack to tree shake it if possible. I read through all of the code in this package and didn't spot any side effects, so I think this should be safe.

For more info:
https://webpack.js.org/guides/tree-shaking/
https://developers.google.com/web/fundamentals/performance/optimizing-javascript/tree-shaking

[coveralls]

Coverage remained the same at 98.552% when pulling aa7375d on ericjeney:patch-1 into a68dfba on fb55:master.

[ericjeney]

See also:
fb55/entities#211
fb55/domutils#222
cheeriojs/dom-serializer#97
fb55/domelementtype#12
fb55/domhandler#232

[fb55]

Thanks a lot @ericjeney, for this, as well as all the other PRs.