Codemod for `template-only-glimmer-components` doesn't support Addons

It appears that the codemod implementation only runs against components in the `app/` directory, which will not work for addons, which store their components in the `addon/` directory and may not re-export them at all (since doing so is dangerous and a bad pattern). In order for addons to successfully enable this feature, the codemod needs to be able to create backing classes for addon components.

https://github.com/emberjs/ember-optional-features/blob/master/features/template-only-glimmer-components.js#L55