Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

tpl/transform/unmarshal.go

@@ -72,7 +72,7 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 		}
 
 		return ns.cache.GetOrCreate(key, func() (any, error) {
-			f := metadecoders.FormatFromMediaType(r.MediaType())
+			f := metadecoders.FormatFromStrings(r.MediaType().Suffixes()...)
 			if f == "" {
 				return nil, fmt.Errorf("MIME %q not supported", r.MediaType())
 			}