Fix build error for consuming apps, caused by (set) helper

[roomman]

My consuming app was throwing the following error, using this addon with the <Transition> component:

Build Error (PackagerRunner) in node_modules/ember-headlessui/components/transition.hbs

Module Error (from .../node_modules/thread-loader/dist/cjs.js):
Missing helper: set in node_modules/ember-headlessui/components/transition.hbs

This PR fixes the issue by moving the set helper module into the package dependencies.

It also upgrades from the deprecated ember-simple-set-helper to ember-set-helper, and includes the required refactoring.

All tests are passing.

[alexlafroscia]

This is awesome; thanks for making this PR!

I had noticed over the weekend that this package was using ember-simple-set-helper but didn't realize that it was used inside the addon tree and not actually set as a dependency. I really appreciate the clean-up here!

[alexlafroscia]

CI doesn't run until approved for first-time contributors, but I just kicked it off. Once it's passing I'll merge and cut a release with this fix!