Improve tree-shaking

[kinday]

Hi there,

I’ve had a quick glance at code and could make a PR, but feels better to ask maintainers first.

There is non-standard (yet widely supported) sideEffect field which, if present in package.json, allows build tools (example from Webpack docs) to better handle tree-shaking.

So, the code (at least of universal-cookie package) seems to be side effect free, but I’d like to get confirmation from you. If you haven’t ever dealt with sideEffects field, in this case we speak about side effects on module evaluation (i.e. import or require).

I tried to cover essential parts, but if further clarification is needed, feel free to ask me.

[eXon]

Yes I'm sure we can add this. Good idea!

[eXon]

Done with v4.0.3