[API] Implement birthday setting

src/api/models/user.ts

@@ -8,3 +8,7 @@ export default collection;
 export function validateUsername(username: string): boolean {
 	return /^[a-zA-Z0-9\-]{3,20}$/.test(username);
 }
+
+export function isValidBirthday(birthday: string): boolean {
+	return /^([0-9]{4})\-([0-9]{2})-([0-9]{2})$/.test(birthday);
+}