Rolling rolled up application.

Vanilla Rollup can only support the ESM format. So one can either include rollup-plugin-commonjs for upstream dependency. Or generate both UMD and ESM bundles for the base lib.

1. yarn rollup creates ./src/index_rolledUp.js and ./src/index_rolledUp.mjs from ./src/index.js
2. ./src/indexSquared.js uses ./src/index_rolledUp.mjs. Rollup will default to .mjs whenever available.
3. yarn rollupSquared to bundle ./dist/index.js

All based on this answer.

Thanks!