Use transparent CORS proxy in Blueprint Builder

[brandonpayton]

Motivation for the change, related issues

The Blueprint builder doesn't use the CORS proxy transparently like the web app does. The reason is that the builder does not pass a CORS proxy URL to startPlaygroundWeb().

Implementation details

This PR does a liberal conversion of builder.js to builder.ts and imports the virtual vite module virtual:cors-proxy-url to obtain the right CORS proxy URL depending on target.

Testing Instructions (or ideally a Blueprint)

• npm run build
• Serve dist/packages/playground/wasm-wordpress-net via a local web server
• Navigate to the Blueprint builder on the server
• Run the following Blueprint and confirm that the Nautilus theme is installed and activated:

{
  "landingPage": "/wp-admin/themes.php",
  "steps": [
    {
      "step": "login",
      "username": "admin"
    },
    {
      "step": "installTheme",
      "themeZipFile": {
        "resource": "url",
        "url": "https://codeload.github.com/ndiego/nautilus/zip/refs/heads/main"
      }
    }
  ]
}

• Play with Blueprint Builder and confirm that all appears to be in working order.