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

media/config.go

@@ -0,0 +1,139 @@
+// 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 media
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"sort"
+	"strings"
+
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/config"
+
+	"github.com/mitchellh/mapstructure"
+	"github.com/spf13/cast"
+)
+
+// DefaultTypes is the default media types supported by Hugo.
+var DefaultTypes Types
+
+func init() {
+
+	ns, err := DecodeTypes(nil)
+	if err != nil {
+		panic(err)
+	}
+	DefaultTypes = ns.Config
+
+	// Initialize the Builtin types with values from DefaultTypes.
+	v := reflect.ValueOf(&Builtin).Elem()
+	for i := 0; i < v.NumField(); i++ {
+		f := v.Field(i)
+		builtinType := f.Interface().(Type)
+		defaultType, found := DefaultTypes.GetByType(builtinType.Type)
+		if !found {
+			panic(errors.New("missing default type for builtin type: " + builtinType.Type))
+		}
+		f.Set(reflect.ValueOf(defaultType))
+	}
+}
+
+// Hold the configuration for a given media type.
+type MediaTypeConfig struct {
+	// The file suffixes used for this media type.
+	Suffixes []string
+	// Delimiter used before suffix.
+	Delimiter string
+}
+
+// DecodeTypes decodes the given map of media types.
+func DecodeTypes(in map[string]any) (*config.ConfigNamespace[map[string]MediaTypeConfig, Types], error) {
+
+	buildConfig := func(v any) (Types, any, error) {
+		m, err := maps.ToStringMapE(v)
+		if err != nil {
+			return nil, nil, err
+		}
+		if m == nil {
+			m = map[string]any{}
+		}
+		m = maps.CleanConfigStringMap(m)
+		// Merge with defaults.
+		maps.MergeShallow(m, defaultMediaTypesConfig)
+
+		var types Types
+
+		for k, v := range m {
+			mediaType, err := FromString(k)
+			if err != nil {
+				return nil, nil, err
+			}
+			if err := mapstructure.WeakDecode(v, &mediaType); err != nil {
+				return nil, nil, err
+			}
+			mm := maps.ToStringMap(v)
+			suffixes, found := maps.LookupEqualFold(mm, "suffixes")
+			if found {
+				mediaType.SuffixesCSV = strings.TrimSpace(strings.ToLower(strings.Join(cast.ToStringSlice(suffixes), ",")))
+			}
+			if mediaType.SuffixesCSV != "" && mediaType.Delimiter == "" {
+				mediaType.Delimiter = DefaultDelimiter
+			}
+			InitMediaType(&mediaType)
+			types = append(types, mediaType)
+		}
+
+		sort.Sort(types)
+
+		return types, m, nil
+	}
+
+	ns, err := config.DecodeNamespace[map[string]MediaTypeConfig](in, buildConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decode media types: %w", err)
+	}
+	return ns, nil
+
+}
+
+func suffixIsRemoved() error {
+	return errors.New(`MediaType.Suffix is removed. Before Hugo 0.44 this was used both to set a custom file suffix and as way
+to augment the mediatype definition (what you see after the "+", e.g. "image/svg+xml").
+
+This had its limitations. For one, it was only possible with one file extension per MIME type.
+
+Now you can specify multiple file suffixes using "suffixes", but you need to specify the full MIME type
+identifier:
+
+[mediaTypes]
+[mediaTypes."image/svg+xml"]
+suffixes = ["svg", "abc" ]
+
+In most cases, it will be enough to just change:
+
+[mediaTypes]
+[mediaTypes."my/custom-mediatype"]
+suffix = "txt"
+
+To:
+
+[mediaTypes]
+[mediaTypes."my/custom-mediatype"]
+suffixes = ["txt"]
+
+Note that you can still get the Media Type's suffix from a template: {{ $mediaType.Suffix }}. But this will now map to the MIME type filename.
+`)
+}