Load monaco from packages not CDN

[jjspace]

Description

By default @monaco-editor/react loads the monaco library from a CDN (through monaco-loader). Normally this would be fine but it means Sandcastle would be unusable when not connected to an external network.

This PR sets up the loader config to make sure we use the local packaged version of monaco so Sandcastle works offline and in isolated networks.
The config is set up using the guidance in the monaco-editor/react readme for vite

I also copied over the Offline sandcastle from #12752 for testing

Issue number and link

Part of #12566

Testing plan

• run npm run dev from inside the sandcastle package or npm run build-sandcastle and npm start from the root package
• Open the offline sandcastle demo http://localhost:5173/index.html?id=offline or http://localhost:8080/Apps/Sandcastle2/index.html?id=offline depending how you started it
• Make sure the code editor loaded and works as expected
• Open the DevTools network tab and filter by -url:localhost to make sure there are no external requests
• Turn off your system wifi/network and refresh the example. Make sure it still works.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Awesome. Looks great, and I can confirm that no cdn assets are being requested.

Thanks @jjspace!