Add scheduleTask and runWorkLoop to public API (#48284)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48284

Changelog: [internal]

We recently prevented tasks that run via `runTask` to synchronously call `runTask` because the execution is synchronous and we can't nest tasks in the event loop (D67107664 / https://github.com/facebook/react-native/pull/48235).

This adds `scheduleTask` to schedule tasks (also within tasks) with the right expectations (the task will not run synchronously on that call but at the right time).

It also adds `runWorkLoop` so we can run scheduled tasks if they're not scheduled from an already running task.

Reviewed By: javache

Differential Revision: D67275518

fbshipit-source-id: acde0093802fbcb7083334f2c0247b37b759a6b1