Investigate how to stop building @bufferapp/ui with commonjs

[msanroman]

What

• Remove commonjs-related build steps.
• Make sure we are able to export independently any component (without bringing in the whole library).
• Test:
  • Run https://www.npmjs.com/package/webpack-bundle-analyzer on a product repo (best to try with bufferapp/buffer-engage-client).
  • Install beta version with these changes on that repo.
  • Run the bundle analyzer again: we should see a decrease in size regarding what's brought by bufferapp/ui

Why
We have a Babel plugin called @babel/plugin-transform-modules-commonjs (https://babeljs.io/docs/en/babel-plugin-transform-modules-commonjs) transforming ECMAScript modules to CommonJS. Our bundle is much bigger, and we can't use tree shaking (we could have a third-party webpack plugin on top of that to get tree shaking but doesn't seem efficient)

[hamstu]

Yes! 👏 👏 👏