Add prefetch and nonce to assets/1

[osbre]

One feature I miss coming from Laravel is their Vite::prefetch() (laravel/framework#52462).

It downloads lazily loaded chunks in the background once the page has loaded. With Inertia, that means the next page's code is already cached when you navigate.

<PhoenixVite.Components.assets
  ...
  prefetch={[concurrency: 3]}
/>

• prefetch downloads them all at once
• prefetch={[concurrency: 3]} downloads three at a time
• prefetch={[event: "app:ready"]} waits for a custom window event instead of load

This PR also adds nonce to assets/1 as a separate commit, since it's useful on its own for a strict CSP.