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

create/content.go

@@ -340,7 +340,7 @@ func (b *contentBuilder) mapArcheTypeDir() error {
 }
 
 func (b *contentBuilder) openInEditorIfConfigured(filename string) error {
-	editor := b.h.Cfg.GetString("newContentEditor")
+	editor := b.h.Conf.NewContentEditor()
 	if editor == "" {
 		return nil
 	}