This leverages [fetch
priority](https://developer.mozilla.org/en-US/docs/Web/API/Request/Request#priority)
to ensure automatic prefetching as a result of visiting a page is sent
with "low" priority, to signal to the browser it can prioritize more
important work necessary for rendering the page.
A "temporary" prefetch (ie one that is created when there wasn't an
existing prefetch cache entry on navigation) will use a "high" priority
because it's critical to the navigation event.
All other cases will be "auto" which is the current default.
aa832c3b2d