Fix preload link headers not working with JavaScript modules

[MadLittleMods]

Follow-up to #171 and #175 where they broke because we went from scripts to modules.

Part of #132

Before this PR, we were seeing these warning in the Chrome devtools console:

A preload for 'foo' is found, but is not used because the request credentials mode does not match. Consider taking a look at crossorigin attribute.

This is caused by a credentials mode mismatch between the <script type="module"> tag and the Link header. A <script type="module"> with no crossorigin attribute indicates a credentials mode of omit and a naive Link: </foo-url>; rel=preload; as=script; has a default credentials mode of same-origin, hence the mismatch and warning we're seeing.

We could set the credentials mode to match using Link: </foo-url>; rel=preload; as=script; omit but there is an even better option! We can use the dedicated Link: </foo-url>; rel=modulepreload link type which not only downloads and puts the the file in the cache like a normal preload but the browser also knows it's a JavaScript module now and can parse/compile it so it's ready to go.

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Future consideration: Adding nopush to preload link headers. Many servers initiate an HTTP/2 Server Push when they encounter a preload link in HTTP header form otherwise. Do we want/care about that (or maybe we don't)? (mentioned in https://medium.com/reloading/preload-prefetch-and-priorities-in-chrome-776165961bbf#6f54)

Dev notes

Preload Link headers:

• https://medium.com/reloading/preload-prefetch-and-priorities-in-chrome-776165961bbf#6f54
• https://html.spec.whatwg.org/multipage/links.html#link-type-preload
• https://www.smashingmagazine.com/2016/02/preload-what-is-it-good-for/#headers
• https://developer.chrome.com/blog/modulepreload/#ok-so-why-doesnt-link-relpreload-work-for-modules

Cloudflare:

• https://developers.cloudflare.com/cache/about/early-hints/
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link
• https://blog.cloudflare.com/http-2-server-push-with-multiple-assets-per-link-header/