migrate to "export default"

also, set no-use-before-define in linter

src/validator.js

@@ -1,8 +1,5 @@
 import assert from 'node:assert';
 
-export {
-    isEmail
-};
 
 // this currently does not match: "john.doe"@example.com, user@[192.168.1.1], john.doe(comment)@example.com or 用户@例子.世界
 function isEmail(email) {
@@ -13,3 +10,6 @@ function isEmail(email) {
   return emailRegex.test(email);
 }
 
+export default {
+    isEmail
+};