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.go

@@ -0,0 +1,57 @@
+// 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 (
+	"fmt"
+	"strings"
+
+	"github.com/gohugoio/hugo/compare"
+)
+
+var _ compare.Eqer = StringEqualFold("")
+
+// StringEqualFold is a string that implements the compare.Eqer interface and considers
+// two strings equal if they are equal when folded to lower case.
+// The compare.Eqer interface is used in Hugo to compare values in templates (e.g. using the eq template function).
+type StringEqualFold string
+
+func (s StringEqualFold) EqualFold(s2 string) bool {
+	return strings.EqualFold(string(s), s2)
+}
+
+func (s StringEqualFold) String() string {
+	return string(s)
+}
+
+func (s StringEqualFold) Eq(s2 any) bool {
+	switch ss := s2.(type) {
+	case string:
+		return s.EqualFold(ss)
+	case fmt.Stringer:
+		return s.EqualFold(ss.String())
+	}
+
+	return false
+}
+
+// EqualAny returns whether a string is equal to any of the given strings.
+func EqualAny(a string, b ...string) bool {
+	for _, s := range b {
+		if a == s {
+			return true
+		}
+	}
+	return false
+}