Missing dependency on @ember/string

[bjornharrtell]

I suspect this is cause of sudden "Could not find module @ember/string imported from ember-modal-dialog/components/modal-dialog", after a transitive dependency upgrade. As of yet unable to find out which transitive dep is causing the break.. but nevertheless, I think it's an error that ember-modal-dialog doesn't declare that it has dependency on @ember/string. Similar problem as emberjs/ember-inflector#501.

[bjornharrtell]

I found that the break occurs when upgrading @ember/string from 3.0.0 to 4.0.0.

[lukemelia]

Addressed in #403