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)

src/workflows/index.ts

@@ -0,0 +1,11 @@
+import { sendEmailWorkflow } from './sendEmailWorkflow.js'
+
+/**
+ * All mailing-related workflows that get registered with Payload
+ */
+export const mailingWorkflows = [
+  sendEmailWorkflow,
+]
+
+// Re-export everything from individual workflow files
+export * from './sendEmailWorkflow.js'