Replace the contentFor hook with a component

[Windvis]

At the moment the addon adds an element to the final html file through a contentFor hook.

While it's nice to provide a "no config" setup, it has the downside that apps not using that index.html file won't be able to use the modal components. This is a common issue in the rdfa-editor addons since the editor gets embedded in other applications.

Another downside is that addons pushing content in the final html file isn't a thing for v2 addons and most likely won't ever be.

An alternative would be to create a AuWormhole or AuModalContainer or similar component that will be used as the target.

We could do this in a non-breaking way by deprecating the contentFor hook logic and read a temporary build-time only flag that can be set by apps to silence the deprecation. The deprecation message and contentFor hook logic can then be removed in the v2 release.

[Windvis]

This isn't blocking for Ember v4 support, so we can probably postpone the removal to v3 and add a deprecation for it first.