feat: Add env variables to support exposing `/workflows/demo` route and `/nodes.json` route

[alexgrozav]

Summary

• Introduces VUE_APP_PUBLIC_WORKFLOWS_DEMO_ROUTE env variable for publicly exposing /workflows/demo route
• Prevents loading authenticated resources when exposing /workflows/demo route
• Introduces PUBLIC_NODES_JSON env variable for publicly exposing /nodes.json route

Related tickets and issues

https://linear.app/n8n/issue/PAY-1325/bug-preview-instance-not-accessible

Review / Merge checklist

• [x] PR title and summary are descriptive. Remember, the title automatically goes into the changelog. Use (no-changelog) otherwise. (conventions)
• [x] Docs updated or follow-up ticket created.
• [x] Tests included.

  A bug is not considered fixed, unless a test is added to prevent it from happening again. A feature is not complete without tests.