chore: update to latests Deno and CLI standards

[WilliamBergamin]

Type of change

• [ ] New sample
• [ ] New feature
• [ ] Bug fix
• [ ] Documentation

Summary

This PR aims to update the sample to the Deno 2 standards and the latest CLI standards.

• Move content of slack.json to .slack/hooks.json
• Remove dependency on mock-fetch/
• Import from JSR as much as possible
• Move imports from import-map.json to deno.jsonc

Requirements

• [x] I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• [x] I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• [x] I've read and agree to the Code of Conduct