[change] export ES modules by default

ES modules are the default package export. Commonjs modules are exported
from 'dist/cjs'. Modern bundlers like webpack can consume ES modules.
The addition of the `sideEffects:false` to the `package.json` helps
webpack tree-shaking modules.

scripts/babel/preset.js

@@ -4,6 +4,7 @@ module.exports = {
       'babel-preset-env',
       {
         loose: true,
+        modules: process.env.MODULES || false,
         exclude: ['transform-es2015-typeof-symbol'],
         targets: {
           browsers: [