Script loader: enable resource preloading with rel='preload'.
Add a wp_preload_resources filter that developers can use to add resource preloading.
Preloading helps the browser discover and prioritize important resources earlier during the page load. This ensures that they are available earlier and are less likely to block the page's render.
Props nico23, swissspidy, igrigorik, westonruter, azaozz, furi3r, aristath, spacedmonkey, peterwilsoncc, mihai2u, gziolo.
Fixes #42438.