Add Resource.Data.Artifacts (or something) #15173

I'm very unsure about the naming above (it was the first I came up with).

But this is relevant for css.Build and js.Build. There may be other use cases, but when e.g.

• loading fonts from /assets using url and ESBuild's fileloader.
• the font file gets a hash in its name (ESBuild is doing this) and it get published (copied to public) as part of the css.Build function execution.
• With the above it's not simple/practical to do font preloading (<link rel="preload" href="???" as="font" type="font/woff2" crossorigin="anonymous">), because the output filename is not known.

Ignoring the naming discussion, I suggest that we do:

{{ $css := resources.Get "main.css" | css.Build }}
{{ range $css.Data.Artifacts }}
  {{ if eq .MediaType.MainType "font" }}
    <link rel="preload" href="{{ .RelPermalink }}" as="font" type="{{ .MediaType.Type }}" crossorigin="anonymous">
  {{ end }}
{{ end }}

Note that the above construct should be safe to run on older versions of Hugo ($css.Data.Artifacts will return nil).

The "artifacts" above are already published, so they can only have a sub set of the Resource interface, but I guess it makes sense to keep the familiar method names (this way the "dot" can be passed to link partials):

type Artifact interface {
   resource.ResourceLinksProvider
   resource.MediaTypeProvider
}