Add sendEmail workflow with immediate processing option

- Create sendEmailWorkflow as a Payload workflow alternative to task
- Add processImmediately option (disabled by default) to send emails immediately
- Expose processEmailItem method in MailingService for individual email processing
- Add comprehensive input schema with conditional fields
- Update plugin to register both tasks and workflows
- Add detailed documentation comparing tasks vs workflows
- Includes status tracking and error handling
- Bump version to 0.3.0 (new feature)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-mailing",
-  "version": "0.2.1",
+  "version": "0.3.0",
   "description": "Template-based email system with scheduling and job processing for PayloadCMS",
   "type": "module",
   "main": "dist/index.js",