convert to ESM so we can use yeoman-generator@v7 and greater

We are already dynamically importing `yeoman-test` via #241 .
This will be a `major` version upgrade.

Any users of this lib (which will be made into an ESM) must:
- not use any explicit paths from the main export (unless we explicitly provide this path)
- dynamically import this lib, if used from commonjs

Start by reviewing the package's current main export and the dynamic yeoman-test import introduced by #241. Trace how consumers use explicit export paths and CommonJS, then define the ESM entry-point changes and verify that supported consumers can import the library without relying on undocumented paths.

Written by the indexing model from the issue text.