Externalise scripts and styles when prerendering

Describe the problem

In certain situations it makes sense to externalise inline scripts and styles for CSP purposes (e.g. when building Chrome extensions).

Describe the proposed solution

This only really applies to prerendered pages (there's basically no good way to externalise a dynamically rendered script), so I propose the following interface:

// svelte.config.js
export default {
  kit: {
    prerender: {
      externalise: true
    }
  }
};

This would override the inlineStyleThreshold option. (If externalise isn't descriptive enough, we can bikeshed this.)

This would write out an inline script to the filesystem for each prerendered page, under the appDir (so the script can be cached immutably).

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

The issue does not name files, tests, or entry points. Start by locating the prerendering implementation and the handling of inline scripts and styles, then assess the proposed prerender.externalise option and its interaction with inlineStyleThreshold. Done means prerendered pages use external files under appDir while dynamically rendered scripts remain unaffected.