Adds currency code to line items

Editor assessment

The subject states the correction exactly: currency codes are now added to line items in the Slack notifications, meaning alerts previously showed amounts without currency context. The defect, the data, and the surface are all in one line. Fix is right and the long but accurate medusa-plugin-slack-notification scope pinpoints the package. No body, none needed. Lowercase 'adds' is trivial. A precise, self-sufficient, well-scoped one-line message.