This updates our `yarn next` command to leverage react v18 by default and removes the need for the test require hook/config modifying when testing react 18. There are some fixtures we need to investigate react 18 support in follow-ups:
- `test/integration/client-navigation-a11y`
- `test/integration/critical-css`
- `test/integration/custom-error-page-exception`
- `test/integration/font-optimization`
- AMP specific tests
Implements https://github.com/vercel/next.js/issues/30997 with some minor tweaks to the design:
* The hook is moved to Client Components (e.g. `pages/_app` instead of `pages/_document`). This was a silly oversight in the original design: the hook needs to be called during server prerendering.
* `useFlushEffects` instead of `useFlushEffect` as there isn't a particularly safe way to implement the singular semantics as a Client Component hook given the current implementation of server rendering.
---
Fixes#30997
### Changes
#### Feature
* Adopt `React.lazy` into `next/dynamic`, enable it when `options.suspense` is `true`
* Support `next/dynamic` with `suspense=true` in SSR and SSG
#### Tests
| Scenario | Case | Behavior |
|:----:|:----:|:----:|
| basics | react 17 or 18 by default | dev/build error or pass |
| blocking rendering | `reactRoot: true` + `concurrentFeatures: false` | dev/build pass |
| concurrent rendering | `reactRoot: true` + `concurrentFeatures: true` | dev/build pass |
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
