Move sendEmail to dedicated file for better visibility

- Extract sendEmail function to src/sendEmail.ts as primary module
- Export BaseEmailData and SendEmailOptions interfaces alongside
- Update all imports to use new location
- Add sendEmailDefault export for CommonJS compatibility
- Export parseAndValidateEmails for external utility use
- Updated README to highlight sendEmail as primary export

Breaking change: BaseEmailData and SendEmailOptions now imported from main module, not utils/helpers

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -59,6 +59,7 @@ export default buildConfig({
 ### 2. Send emails with type-safe helper
 
 ```typescript
+// sendEmail is a primary export for easy access
 import { sendEmail } from '@xtr-dev/payload-mailing'
 import { Email } from './payload-types' // Your generated types