Add constant for default phone type (work)

src/MDPhoneTypesSet.php

@@ -17,6 +17,8 @@ declare(strict_types = 1);
  */
 final class MDPhoneTypesSet extends MDValueSet {
 
+    const DEFAULT_PHONE_TYPE = 'work';
+
     const PHONE_TYPES = [
         'work',
         'home',