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

output/config_test.go

@@ -0,0 +1,98 @@
+// 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 output
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/media"
+)
+
+func TestDecodeConfig(t *testing.T) {
+	c := qt.New(t)
+
+	mediaTypes := media.Types{media.Builtin.JSONType, media.Builtin.XMLType}
+
+	tests := []struct {
+		name        string
+		m           map[string]any
+		shouldError bool
+		assert      func(t *testing.T, name string, f Formats)
+	}{
+		{
+			"Redefine JSON",
+			map[string]any{
+				"json": map[string]any{
+					"baseName":    "myindex",
+					"isPlainText": "false",
+				},
+			},
+			false,
+			func(t *testing.T, name string, f Formats) {
+				msg := qt.Commentf(name)
+				c.Assert(len(f), qt.Equals, len(DefaultFormats), msg)
+				json, _ := f.GetByName("JSON")
+				c.Assert(json.BaseName, qt.Equals, "myindex")
+				c.Assert(json.MediaType, qt.Equals, media.Builtin.JSONType)
+				c.Assert(json.IsPlainText, qt.Equals, false)
+			},
+		},
+		{
+			"Add XML format with string as mediatype",
+			map[string]any{
+				"MYXMLFORMAT": map[string]any{
+					"baseName":  "myxml",
+					"mediaType": "application/xml",
+				},
+			},
+			false,
+			func(t *testing.T, name string, f Formats) {
+				c.Assert(len(f), qt.Equals, len(DefaultFormats)+1)
+				xml, found := f.GetByName("MYXMLFORMAT")
+				c.Assert(found, qt.Equals, true)
+				c.Assert(xml.BaseName, qt.Equals, "myxml")
+				c.Assert(xml.MediaType, qt.Equals, media.Builtin.XMLType)
+
+				// Verify that we haven't changed the DefaultFormats slice.
+				json, _ := f.GetByName("JSON")
+				c.Assert(json.BaseName, qt.Equals, "index")
+			},
+		},
+		{
+			"Add format unknown mediatype",
+			map[string]any{
+				"MYINVALID": map[string]any{
+					"baseName":  "mymy",
+					"mediaType": "application/hugo",
+				},
+			},
+			true,
+			func(t *testing.T, name string, f Formats) {
+			},
+		},
+	}
+
+	for _, test := range tests {
+		result, err := DecodeConfig(mediaTypes, test.m)
+		msg := qt.Commentf(test.name)
+
+		if test.shouldError {
+			c.Assert(err, qt.Not(qt.IsNil), msg)
+		} else {
+			c.Assert(err, qt.IsNil, msg)
+			test.assert(t, test.name, result.Config)
+		}
+	}
+}