Register maker is added. (#149)

# Describe Request

Register maker is added.

# Change Type

New code.

v2/maker.go

@@ -46,6 +46,11 @@ var makers = map[string]MakeCheckFunc{
 	nameURLUnescape:  makeURLUnescape,
 }
 
+// RegisterMaker registers a new maker function with the given name.
+func RegisterMaker(name string, maker MakeCheckFunc) {
+	makers[name] = maker
+}
+
 // makeChecks take a checker config and returns the check functions.
 func makeChecks(config string) []CheckFunc[reflect.Value] {
 	fields := strings.Fields(config)

v2/maker_test.go

@@ -6,6 +6,8 @@
 package v2_test
 
 import (
+	"fmt"
+	"reflect"
 	"testing"
 
 	v2 "github.com/cinar/checker/v2"
@@ -24,3 +26,51 @@ func TestMakeCheckersUnknown(t *testing.T) {
 
 	v2.CheckStruct(person)
 }
+
+func ExampleRegisterMaker() {
+	v2.RegisterMaker("is-fruit", func(params string) v2.CheckFunc[reflect.Value] {
+		return func(value reflect.Value) (reflect.Value, error) {
+			stringValue := value.Interface().(string)
+
+			if stringValue == "apple" || stringValue == "banana" {
+				return value, nil
+			}
+
+			return value, v2.NewCheckError("NOT_FRUIT")
+		}
+	})
+
+	type Item struct {
+		Name string `checkers:"is-fruit"`
+	}
+
+	person := &Item{
+		Name: "banana",
+	}
+
+	err, ok := v2.CheckStruct(person)
+	if !ok {
+		fmt.Println(err)
+	}
+}
+
+func TestRegisterMaker(t *testing.T) {
+	v2.RegisterMaker("unknown", func(params string) v2.CheckFunc[reflect.Value] {
+		return func(value reflect.Value) (reflect.Value, error) {
+			return value, nil
+		}
+	})
+
+	type Person struct {
+		Name string `checkers:"unknown"`
+	}
+
+	person := &Person{
+		Name: "Onur",
+	}
+
+	_, ok := v2.CheckStruct(person)
+	if !ok {
+		t.Fatal("expected valid")
+	}
+}