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/allconfig/load_test.go

@@ -0,0 +1,67 @@
+package allconfig
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+)
+
+func BenchmarkLoad(b *testing.B) {
+	tempDir := b.TempDir()
+	configFilename := filepath.Join(tempDir, "hugo.toml")
+	config := `
+baseURL = "https://example.com"
+defaultContentLanguage = 'en'
+
+[module]
+[[module.mounts]]
+source = 'content/en'
+target = 'content/en'
+lang = 'en'
+[[module.mounts]]
+source = 'content/nn'
+target = 'content/nn'
+lang = 'nn'
+[[module.mounts]]
+source = 'content/no'
+target = 'content/no'
+lang = 'no'
+[[module.mounts]]
+source = 'content/sv'
+target = 'content/sv'
+lang = 'sv'
+[[module.mounts]]
+source = 'layouts'
+target = 'layouts'
+
+[languages]
+[languages.en]
+title = "English"
+weight = 1
+[languages.nn]
+title = "Nynorsk"
+weight = 2
+[languages.no]
+title = "Norsk"
+weight = 3
+[languages.sv]
+title = "Svenska"
+weight = 4
+`
+	if err := os.WriteFile(configFilename, []byte(config), 0666); err != nil {
+		b.Fatal(err)
+	}
+	d := ConfigSourceDescriptor{
+		Fs:       afero.NewOsFs(),
+		Filename: configFilename,
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := LoadConfig(d)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}