With the availability of the `@ember/destroyables` API, we no longer
need these to be handled via lifecycle hooks. Users previously set up
their own `willDestroy` hook, and it got called for them:

    import Modifier from 'ember-modifier';

    class MyModifier extends Modifier {
      willDestroy() {
        // ...
      }
    }

Now, like with other classes in modern Ember/Glimmer, this can stop
being a "lifecycle hook" and simply use the `registerDestructor` API:

    import Modifier from 'ember-modifier';
    import { registerDestructor } from '@ember/destroyable';

    class MyModifier extends Modifier {
      constructor(owner, args) {
        super(owner, args);
        registerDestructor(this, () => {
          // ...
        });
      }
    }

Similarly, instead of checking the `.isDestroyed` or `.isDestroying`
properties on the instance, users can import the correspondingly-named
functions from `@ember/destroyables`, e.g. `isDestroyed(instance)`.