feat(notification): implement dynamic Slack component

[betterclever]

Summary

Implemented a robust Slack Notification Component allowing workflows to send dynamic messages via Bot Tokens or Incoming Webhooks.
This PR addresses Feature #2 on the Roadmap ('Slack/Teams Message').

Key Features

• Dual Authentication: Support for both Bot Tokens (API) and Incoming Webhooks.
• Dynamic Templating: Define variables (e.g., `{{time}}`) that are exposed as input ports and interpolated into messages.
• Rich Formatting: Full support for Slack Block Kit JSON with template interpolation.
• Threading: Optional threading support via `thread_ts`.
• UI Integration: Uses the `SimpleVariableListEditor` for intuitive variable definition.
• Categorization: Introduced new `notification` category.

Testing

• Added comprehensive unit tests in `slack.test.ts` covering auth, interpolation, and error handling.
• Validated end-to-end with `.playground/test-slack-e2e.ts`, confirming secret injection, dynamic data flow, and message delivery.