Synchronize non-external HTML assets with publicPath value

[lukeed]

As part of the HTML build step, look at any asset path that is not ^(https?:)?\/\/ and update it so that it's led with the desired publicPath prefix.

Safe to do with asset paths like href="/styles.css"" since that's assuming root-resolver.
Less safe to manipulate href="styles.css" since that may need to be a location-based resolver, but I think it's okay for freshie to say that, as the root HTML template, it should only be absolute URLs... since that template is/can be used to render any page.

---

Related: lukeed/svelte-ssr-worker#2 (comment)