Convert to V2 Addon

[NullVoxPopuli]

We already require ember-auto-import@v2 so when this PR is complete, it'll be a non-breaking change.

How we got here (in order)

1. Move ember-modifier to dependencies #127
2. Remove @tailwindcss/ui #147
3. Switch to pnpm #148
4. Convert to monorepo #152
5. Move tests to own app #157
6. and now this PR! (Convert to V2 Addon #144)

How do you review something like this? (It's still a big PR)
Points of interest:

• the ci workflow is now generated from the root ci.yml, using: https://github.com/NullVoxPopuli/ember-ci-update
• the rollup.config presently makes all files publicEntrypoints. We'll likely want to shift to gjs/gts and private component/modifier imports to reduce those publicEntrypoints -- because there is def stuff we don't want to be public API as most of this addon is to support the publicEntrypoints

Things to keep in mind

• v2 addons + monorepos simulate real npm package + app consumption as close as possible
• we don't have any sort of deploy preview set up or anything, so you may want to see if the docs app boots up and looks fine
• if all the status checks are green, we should be good to go