Needs `as` attribute

See here: https://www.smashingmagazine.com/2016/02/preload-what-is-it-good-for/#loading-of-late-discovered-resources
If the `as` attribute is omitted from the preload, it has lower priority or isn't even used at all (Chrome shows: `was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.`

The issue reports that generated preload hints omit the `as` attribute and links to a reference explaining the impact. Start by tracing how resource hints are generated in this webpack plugin; done means emitted preload markup includes the appropriate `as` value and avoids the browser warning described in the issue.

Written by the indexing model from the issue text.