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

resources/resource/resources.go

@@ -144,6 +144,7 @@ func (r Resources) MergeByLanguage(r2 Resources) Resources {
 
 // MergeByLanguageInterface is the generic version of MergeByLanguage. It
 // is here just so it can be called from the tpl package.
+// This is for internal use.
 func (r Resources) MergeByLanguageInterface(in any) (any, error) {
 	r2, ok := in.(Resources)
 	if !ok {