[Umbrella] Releasing Suspense

Let's use this issue to track the remaining tasks for releasing Suspense to open source. **Last updated: March 24, 2022** **Blog post: [The Plan for React 18](https://reactjs.org/blog/2021/06/08/the-plan-for-react-18.html)** ## Completed: React 16 - [x] Release `<Suspense>` with `React.lazy` for client-side lazy loading ## Completed: [React 18 Alpha](https://reactjs.org/blog/2021/06/08/the-plan-for-react-18.html) - [x] Implement concurrent rendering, which is a prerequisite to everything else. - [x] Fix [fundamental flaws](https://github.com/facebook/react/pull/18796) in the concurrency model that made the behavior difficult to understand and caused many bugs. - [x] Rewrite [how React traverses the tree](https://github.com/facebook/react/pull/19261) to unblock fixing Suspense quirks. - [x] Redesign how [React integrates with the scheduler](https://github.com/facebook/react/pull/19121) to simplify the model, fix bugs, and prepare for native browser scheduling. - [x] [Fix `<Suspense>` quirks](https://github.com/reactwg/react-18/discussions/7): Previously, effects would fire inside a suspended tree too early. For example, you would see an effect from a component that's still hidden behind a placeholder. Now effects will run only _after_ the content has been revealed. We expect this to fix existing application code bugs. - [x] [Hiding and showing existing content should re-fire layout effects](https://github.com/reactwg/react-18/discussions/31): If a component that's already visible suspends, we show a placeholder, and later show it again. However, there was no way for the component to know that it was hidden or shown. For example, a tooltip component measuring its screen position would get incorrect measurements while it's hidden. Now we fire `useLayoutEffect` cleanup (same as `componentWillUnmount`) on "hide", and `useLayoutEffect` setup (same as `componentDidMount`) on "show". We expect this to fix existing application and library code bugs. - [x] [`<Suspense>` on th…