Change package main file to `dist`, alternative `module` for webpack/rollup

[euxn23]

I think package.json's main field should be built file. (can be used in vanilla js). Now for webpack/rollup, module field is available, so I edited package.json about them.

FYI: What is the “module” package.json field for?

[Disorrder]

Well, Webpack recommends to import code from src, and not recommend to use minified files. Unfortunately I forgot where I saw it to proof But! We can't use sources because of #185 😠