refactor:Configuration 클래스에 싱글톤 패턴 적용

Author: whrjsgml0000

src/main/java/calculator/Application.java

@@ -4,7 +4,7 @@
 import calculator.controller.CalculatorController;
 
 public class Application {
-    private static final Configuration configuration = new Configuration();
+    private static final Configuration configuration = Configuration.getInstance();
 
     public static void main(String[] args) {
         CalculatorController calculatorController = configuration.getCalculatorController();

src/main/java/calculator/config/Configuration.java

@@ -4,10 +4,14 @@
 import calculator.service.CalculatorService;
 
 public class Configuration {
-    private final CalculatorController calculatorController;
+    private static final Configuration configuration = new Configuration();
+    private final CalculatorController calculatorController = new CalculatorController();
 
-    public Configuration() {
-        this.calculatorController = new CalculatorController();
+    private Configuration() {
+    }
+
+    public static Configuration getInstance() {
+        return configuration;
     }
 
     public CalculatorController getCalculatorController() {