Tracking issue for making this blueprint the default

- [ ] update ember-cli guides
- [x] rename testem.js to testem.cjs
- [x] make sure testem.cjs specifies the tests' output folder. [done here](https://github.com/ember-cli/ember-addon-blueprint/blob/main/files/testem.cjs#L6)
- [ ] provide distinct dev vs publish tsconfigs
- [ ] re-add multiple ember version matrix testing (@embroider/try or newer lightweight ember-try) https://github.com/ember-cli/ember-addon-blueprint/pull/41
- [ ] type=module https://github.com/ember-cli/ember-addon-blueprint/pull/6
- [ ] add a demo index.html (and remove overridden entrypoints config)
- [ ] publint and/or are-the-types-wrong
- [ ] add skip-git flag
- [ ] add skip-npm flag
- [ ] ensure there's a clear strategy for self-imports in tests (package.json points at dist, tests build directly against src, so node's default self-importing support will not work). Or explain that you should use subpath imports instead.
- https://github.com/ember-cli/ember-addon-blueprint/pull/47
- [x] testem should use `--headless=new` https://github.com/ember-cli/ember-addon-blueprint/pull/45