This will allow us to extend the deprecation period for this-property-fallback beyond Ember 4.x, to give more time for plugin developers to update their templates.
It's backward compatible so still supports our 3.28 ember-source.
The visible change is finally getting rid of this message:
```
WARNING: Node v18.12.0 is not tested against Ember CLI on your platform. We recommend that you use the most-recent "Active LTS" version of Node.js. See https://git.io/v7S5n for details.
```
---
`@ember/string` dependency is added for future compatibility. See: https://github.com/ember-cli/ember-cli/pull/10125
---
`tests/helpers/index.js` is unused for now, but is a nice pattern. We could move some of our test setup into local `setupApplicationTest/setupRenderingTest/setupTest` helpers.
Co-authored-by: David Taylor <david@taylorhq.com>
