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

hugofs/rootmapping_fs_test.go

@@ -30,7 +30,7 @@ import (
 
 func TestLanguageRootMapping(t *testing.T) {
 	c := qt.New(t)
-	v := config.NewWithTestDefaults()
+	v := config.New()
 	v.Set("contentDir", "content")
 
 	fs := NewBaseFileDecorator(afero.NewMemMapFs())