add Publish::Slack

[bonsai]

This pull request introduces a new plugin, Publish::Slack, to integrate Slack notifications into the Plagger framework. The plugin allows publishing entry updates to a Slack channel, including metadata such as title, author, date, tags, and a link. It ensures robust handling of data and errors during Slack message construction and posting.

New Plugin Integration:

• Slack Notification Plugin (lib/Plagger/Plugin/Publish/Slack.pm):
  • Added a new plugin, Publish::Slack, that sends entry updates to Slack using a webhook URL. It includes configurable settings for channel, username, and icon_emoji.
  • Implements robust error handling for JSON encoding and Slack API responses, with fallback mechanisms to ensure message delivery.
  • Constructs Slack messages with rich metadata, including title, author, date, tags, and a link, using attachments for better formatting.