postbuild.sh is the wrong approach

[internettrans]

Instead of using bash and sed, we should use babel cli to convert files to esm and system.register format.

• https://github.com/prateekbh/babel-esm-plugin
• https://babeljs.io/docs/en/babel-cli
• babel src node_modules/crocks esm --source-maps

I will not approve any merge requests or new repos that use bash instead of babel for this. @bennypowers