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/hugo_sites_build_errors_test.go

@@ -337,6 +337,7 @@ minify = true
 		},
 	).BuildE()
 
+	b.Assert(err, qt.IsNotNil)
 	fe := herrors.UnwrapFileError(err)
 	b.Assert(fe, qt.IsNotNil)
 	b.Assert(fe.Position().LineNumber, qt.Equals, 2)