fix(@angular/ssr): export PrerenderFallback

Importing `PrerenderFallback` in a project throws at build time in 19.0.0-rc.2 with:

```ts
✘ [ERROR] No matching export in "node_modules/@angular/ssr/fesm2022/ssr.mjs" for import "PrerenderFallback"

    src/app/app.routes.server.ts:1:9:
      1 │ import { PrerenderFallback, RenderMode, ServerRoute } from '@AnGula...
```

This exports `PrerenderFallback` the same way `RenderMode` is exported to fix the issue.

(cherry picked from commit d77527d)

Author: cexbrayat

packages/angular/ssr/public_api.ts

@@ -12,7 +12,7 @@ export { AngularAppEngine } from './src/app-engine';
 export { createRequestHandler, type RequestHandlerFunction } from './src/handler';
 
 export {
-  type PrerenderFallback,
+  PrerenderFallback,
   type ServerRoute,
   type ServerRoutesConfigOptions,
   provideServerRoutesConfig,