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

config/namespace.go

@@ -0,0 +1,76 @@
+// 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 config
+
+import (
+	"encoding/json"
+
+	"github.com/gohugoio/hugo/identity"
+)
+
+func DecodeNamespace[S, C any](configSource any, buildConfig func(any) (C, any, error)) (*ConfigNamespace[S, C], error) {
+
+	// Calculate the hash of the input (not including any defaults applied later).
+	// This allows us to introduce new config options without breaking the hash.
+	h := identity.HashString(configSource)
+
+	// Build the config
+	c, ext, err := buildConfig(configSource)
+	if err != nil {
+		return nil, err
+	}
+
+	if ext == nil {
+		ext = configSource
+	}
+
+	if ext == nil {
+		panic("ext is nil")
+	}
+
+	ns := &ConfigNamespace[S, C]{
+		SourceStructure: ext,
+		SourceHash:      h,
+		Config:          c,
+	}
+
+	return ns, nil
+}
+
+// ConfigNamespace holds a Hugo configuration namespace.
+// The construct looks a little odd, but it's built to make the configuration elements
+// both self-documenting and contained in a common structure.
+type ConfigNamespace[S, C any] struct {
+	// SourceStructure represents the source configuration with any defaults applied.
+	// This is used for documentation and printing of the configuration setup to the user.
+	SourceStructure any
+
+	// SourceHash is a hash of the source configuration before any defaults gets applied.
+	SourceHash string
+
+	// Config is the final configuration as used by Hugo.
+	Config C
+}
+
+// MarshalJSON marshals the source structure.
+func (ns *ConfigNamespace[S, C]) MarshalJSON() ([]byte, error) {
+	return json.Marshal(ns.SourceStructure)
+}
+
+// Signature returns the signature of the source structure.
+// Note that this is for documentation purposes only and SourceStructure may not always be cast to S (it's usually just a map).
+func (ns *ConfigNamespace[S, C]) Signature() S {
+	var s S
+	return s
+}