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

common/hstrings/strings_test.go

@@ -0,0 +1,36 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hstrings
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestStringEqualFold(t *testing.T) {
+	c := qt.New(t)
+
+	s1 := "A"
+	s2 := "a"
+
+	c.Assert(StringEqualFold(s1).EqualFold(s2), qt.Equals, true)
+	c.Assert(StringEqualFold(s1).EqualFold(s1), qt.Equals, true)
+	c.Assert(StringEqualFold(s2).EqualFold(s1), qt.Equals, true)
+	c.Assert(StringEqualFold(s2).EqualFold(s2), qt.Equals, true)
+	c.Assert(StringEqualFold(s1).EqualFold("b"), qt.Equals, false)
+	c.Assert(StringEqualFold(s1).Eq(s2), qt.Equals, true)
+	c.Assert(StringEqualFold(s1).Eq("b"), qt.Equals, false)
+
+}