Lint package.json requirements with eslint-plugin-json-files

With @kellyselden's PR here: https://github.com/kellyselden/eslint-plugin-json-files

I think it now makes sense to add some _very important_, yet presently missing lints for:
- package.json
- for all addons
- keywords _must_ have `ember-addon`
- v2 addons
- require an `addon-main` entry and the file must exist
- `ember-addon` (or `ember`) entry must exist
- `version` must be set to `2`
- `addon-main` must be present in `exports`
- `app-js` is optional, but must point at files present in the `exports`
- a the output / `dist` folder must include at least one entry in `exports` (name of `dist` should be configurable)
- if a v2 addon is "browser-only", we don't need an `engines` entry
- forbid `dependencies` (things common in v1 addons)
- ember-cli-htmlbars
- ember-cli-babel
- recommend `peerDependencies` (+ devDeps):
- ember-source