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

parser/metadecoders/format.go

@@ -16,8 +16,6 @@ package metadecoders
 import (
 	"path/filepath"
 	"strings"
-
-	"github.com/gohugoio/hugo/media"
 )
 
 type Format string
@@ -33,6 +31,16 @@ const (
 	XML  Format = "xml"
 )
 
+// FormatFromStrings returns the first non-empty Format from the given strings.
+func FormatFromStrings(ss ...string) Format {
+	for _, s := range ss {
+		if f := FormatFromString(s); f != "" {
+			return f
+		}
+	}
+	return ""
+}
+
 // FormatFromString turns formatStr, typically a file extension without any ".",
 // into a Format. It returns an empty string for unknown formats.
 func FormatFromString(formatStr string) Format {
@@ -59,18 +67,6 @@ func FormatFromString(formatStr string) Format {
 	return ""
 }
 
-// FormatFromMediaType gets the Format given a MIME type, empty string
-// if unknown.
-func FormatFromMediaType(m media.Type) Format {
-	for _, suffix := range m.Suffixes() {
-		if f := FormatFromString(suffix); f != "" {
-			return f
-		}
-	}
-
-	return ""
-}
-
 // FormatFromContentString tries to detect the format (JSON, YAML, TOML or XML)
 // in the given string.
 // It return an empty string if no format could be detected.