use fetch priority for app prefetches

[ztanner]

This leverages fetch 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.

[ztanner]

• #67356 👈
• canary

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[ijjk]

Tests Passed