Support priority on Mail.Options (#34720)

* Support priority on Mail.Options

As stated in this file:
https://github.com/nodemailer/nodemailer/blob/master/lib/mailer/mail-message.js

* coding style errors

Error in nodemailer
Error: /home/travis/build/DefinitelyTyped/DefinitelyTyped/types/nodemailer/lib/mailer/index.d.ts:140:1
ERROR: 140:1   no-trailing-whitespace  trailing whitespace
ERROR: 141:19  typedef-whitespace      expected onespace after colon in property-declaration
ERROR: 141:19  whitespace              missing whitespace

types/nodemailer/lib/mailer/index.d.ts

@@ -137,6 +137,7 @@ declare namespace Mail {
         dkim?: DKIM.Options;
         /** method to normalize header keys for custom caseing */
         normalizeHeaderKey?(key: string): string;
+        priority?: "high"|"normal"|"low";
     }
 
     type PluginFunction = (mail: MailMessage, callback: (err?: Error | null) => void) => void;