fix(@angular/ssr): export PrerenderFallback

[cexbrayat]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

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

✘ [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...

What is the new behavior?

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

Does this PR introduce a breaking change?

• Yes
• No

Other information

[jkrems]

The changes were merged into the following branches: main, 19.0.x

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