Add flight render starting point (#36760)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] 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`
- [ ] 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`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

packages/next/server/view-render.tsx

@@ -219,9 +219,19 @@ export async function renderToHTML(
   const hasConcurrentFeatures = !!runtime
   const pageIsDynamic = isDynamicRoute(pathname)
   const components = Object.keys(ComponentMod.components)
+    .filter((path) => {
+      const { __flight__, __flight_router_path__: routerPath } = query
+      // Rendering part of the page is only allowed for flight data
+      if (__flight__ !== undefined && routerPath) {
+        // TODO: check the actual path
+        const pathLength = path.length
+        return pathLength >= routerPath.length
+      }
+      return true
+    })
     .sort()
-    .map((key) => {
-      const mod = ComponentMod.components[key]()
+    .map((path) => {
+      const mod = ComponentMod.components[path]()
       mod.Component = mod.default || mod
       return mod
     })