Add sideEffects in package.json?

[mindrones]

It would be nice to know if a d3 module has side effects without having to read the code, by inspecting the sideEffects property in package.json.

This would permit to enhance tree shaking by reading that value programmatically and set rollup's treeshake.moduleSideEffects accordingly.

If this makes sense and is doable, this would apply to all of d3 modules (EDIT: especially if you'll do a major version bump across all the modules!)

Thanks!

[mbostock]

Duplicate of d3/d3#3131. This is planned, but please see caveats in that discussion and comments would be welcome.