feat: add ability to provide a prefix for modals

[scottyd980]

I would like to make it possible for modals to live outside of the main components folder. This allows custom prefixes on the name, which adds additional flexibility to where you can put your modals in your folder structure.

This allows something along these lines:

this.modal.open("info-modal", { prefix: "modals/" })

The info-modal can now live inside components/modals/info-modal. With a significant number of modals in an application, it can become unweidly to have a lot of top level modal- modals.

The default fallback returns modal-${name} as it did before.

[scottyd980]

@adrigzr I'm not sure if what I did caused the CI failures - seems like the deps PRs are also failing for a similar reason. It looks like it fails on 3 of the ember try scenarios for possibly unrelated reasons.