refactor: Ensure module preload polyfill is inlined into main bundle

[rschristian]

This has been a fairly long-standing issue in the prerenderer that I've unfortunately just not had the time to track down.

The issue is that Rollup/Vite notices both the web entry chunk & the prerender chunk (containing, at a realistic minimum, rts) both depend on a number of shared modules (preact, amongst others) and so it will extract out those shared modules to a separate chunk. This gives us an app entry module like the following:

import '/index.hash123.js';
function modulePreloadPolyfill() { ... }

You can see this if you load up the Preact docs site, the first JS file that's downloaded is just a static import of our actual app & Vite's modulepreload polyfill -- not ideal.

Vite has always added a modulepreload link with the real bundle path, and with this entry being so tiny I wasn't too concerned with the delay it'd cause, but it probably is a wasted ~20+ms on average which is far from optimal.

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This PR corrects the behavior, ensuring our bundle & the preload polyfill are merged. This means we have to patch the preload polyfill with a window check too, as the polyfill is an IIFE and the prerender chunk now relies on it, but that should be fine.

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