Make `index.html` available in the middleware

It's common to use `middleware` to modify your `index.html` before sending it as the response: 🤔

```ts
import { indexHtml } from 'vite-vercel/server'

export default (req) => {
  const html = indexHtml.replace('<html>', '<html lang="en">')
  return new Response(html)
}
```