Enhance phoneNumberField with country dropdown visibility control

Introduced a CheckBox bound to a new showCountryDropdownProperty() inside SinglePhoneNumberFieldApp. This permits users to toggle visibility of the country dropdown in the phoneNumberField. It enhances UI customization, allowing users to tailor interface to their needs. Code UI was also updated to reflect the dropdown's visibility status.

phonenumberfx-demo/src/main/java/com/dlsc/phonenumberfx/demo/SinglePhoneNumberFieldApp.java

@@ -37,10 +37,13 @@ public void start(Stage stage) {
         CheckBox disableCountryCodeBox = new CheckBox("Disable country dropdown");
         disableCountryCodeBox.selectedProperty().bindBidirectional(field.disableCountryDropdownProperty());
 
+        CheckBox showCountryCodeBox = new CheckBox("Show country dropdown");
+        showCountryCodeBox.selectedProperty().bindBidirectional(field.showCountryDropdownProperty());
+
         CheckBox editableBox = new CheckBox("Editable");
         editableBox.selectedProperty().bindBidirectional(field.editableProperty());
 
-        vBox.getChildren().addAll(new Separator(), clearButton, showExampleBox, countryCodeVisibleBox, disableCountryCodeBox, editableBox);
+        vBox.getChildren().addAll(new Separator(), clearButton, showExampleBox, countryCodeVisibleBox, showCountryCodeBox, disableCountryCodeBox, editableBox);
 
         ScrollPane scrollPane = new ScrollPane();
         scrollPane.setContent(vBox);