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/lang/init.go

@@ -25,7 +25,7 @@ const name = "lang"
 
 func init() {
 	f := func(d *deps.Deps) *internal.TemplateFuncsNamespace {
-		ctx := New(d, langs.GetTranslator(d.Language))
+		ctx := New(d, langs.GetTranslator(d.Conf.Language()))
 
 		ns := &internal.TemplateFuncsNamespace{
 			Name:    name,