"scheduler" package umbrella

### Short term - [x] #13561: Replace the [`__getInteractionsRef` and `__getSubscriberRef` methods](https://github.com/facebook/react/blob/fb88fd9d8c9f72b8e2e7e1ae89652d2a6a707562/packages/schedule/src/Tracking.js#L84-L90) with direct ~~mutable~~ exports (`__interactions` and `__subscriber`) and [update the `ScheduleTracking` UMD fork](https://github.com/facebook/react/blob/master/packages/shared/forks/ScheduleTracking.umd.js). - [x] #13561: Remove `__getInteractionsRef` and `__getSubscriberRef` from the `schedule/tracking` UMD [dev](https://github.com/facebook/react/blob/master/packages/schedule/npm/umd/schedule-tracking.development.js) and [prod](https://github.com/facebook/react/blob/master/packages/schedule/npm/umd/schedule-tracking.production.min.js) bundles since it's not part of the public API. - [x] Fix interaction-tracking reference count bug (#13574) - [ ] Split subscriptions entry point apart from tracking (e.g. break `scheduler/tracking` into `scheduler/tracking` and `scheduler/subscriptions`) ### Long term - [ ] Move `scheduler` to is own git repo (maybe even its own org) prior to a 1.0 release. - [ ] Enable it to be used without a build step by replacing the dev/prod check with a single export. - [ ] Add an ES module entry point as well as the CJS and UMD entry points