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

hugolib/robotstxt_test.go

@@ -28,7 +28,7 @@ const robotTxtTemplate = `User-agent: Googlebot
 func TestRobotsTXTOutput(t *testing.T) {
 	t.Parallel()
 
-	cfg := config.NewWithTestDefaults()
+	cfg := config.New()
 	cfg.Set("baseURL", "http://auth/bub/")
 	cfg.Set("enableRobotsTXT", true)