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

parser/lowercase_camel_json_test.go

@@ -0,0 +1,33 @@
+package parser
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestReplacingJSONMarshaller(t *testing.T) {
+	c := qt.New(t)
+
+	m := map[string]any{
+		"foo":        "bar",
+		"baz":        42,
+		"zeroInt1":   0,
+		"zeroInt2":   0,
+		"zeroFloat":  0.0,
+		"zeroString": "",
+		"zeroBool":   false,
+		"nil":        nil,
+	}
+
+	marshaller := ReplacingJSONMarshaller{
+		Value:       m,
+		KeysToLower: true,
+		OmitEmpty:   true,
+	}
+
+	b, err := marshaller.MarshalJSON()
+	c.Assert(err, qt.IsNil)
+
+	c.Assert(string(b), qt.Equals, `{"baz":42,"foo":"bar"}`)
+}