Add unstable_useFlushEffects hook (vercel#34117)

Implements vercel#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 vercel#30997

errors/client-flush-effects.md

@@ -0,0 +1,24 @@
+# `useFlushEffects` can not be called on the client
+
+#### Why This Error Occurred
+
+The `useFlushEffects` hook was executed while rendering a component on the client, or in another unsupported environment.
+
+#### Possible Ways to Fix It
+
+The `useFlushEffects` hook can only be called while _server rendering a client component_. As a best practice, we recommend creating a wrapper hook:
+
+```jsx
+// lib/use-style-libraries.js
+import { useFlushEffects } from 'next/streaming'
+
+export default function useStyleLibraries() {
+  if (typeof window === 'undefined') {
+    // eslint-disable-next-line react-hooks/rules-of-hooks
+    useFlushEffects([
+      /* ... */
+    ])
+  }
+  /* ... */
+}
+```

errors/manifest.json

@@ -623,6 +623,14 @@
         {
           "title": "opening-an-issue",
           "path": "/errors/opening-an-issue.md"
+        },
+        {
+          "title": "multiple-flush-effects",
+          "path": "/errors/multiple-flush-effects.md"
+        },
+        {
+          "title": "client-flush-effects",
+          "path": "/errors/client-flush-effects.md"
         }
       ]
     }

errors/multiple-flush-effects.md

@@ -0,0 +1,9 @@
+# The `useFlushEffects` hook cannot be used more than once.
+
+#### Why This Error Occurred
+
+The `useFlushEffects` hook is being used more than once while a page is being rendered.
+
+#### Possible Ways to Fix It
+
+The `useFlushEffects` hook should only be called inside the body of the `pages/_app` component, before returning any `<Suspense>` boundaries. Restructure your application to prevent extraneous calls.

packages/next/client/streaming/index.ts

@@ -1,2 +1,3 @@
 export { useRefreshRoot as unstable_useRefreshRoot } from './refresh'
 export { useWebVitalsReport as unstable_useWebVitalsReport } from './vitals'
+export { useFlushEffects as unstable_useFlushEffects } from '../../shared/lib/flush-effects'

packages/next/server/render-result.ts

@@ -1,9 +1,9 @@
 import type { ServerResponse } from 'http'
 
 export default class RenderResult {
-  _result: string | ReadableStream
+  _result: string | ReadableStream<Uint8Array>
 
-  constructor(response: string | ReadableStream) {
+  constructor(response: string | ReadableStream<Uint8Array>) {
     this._result = response
   }