chore: Add .github/CODEOWNERS file

[mwbrooks]

Summary

This pull request adds a .github/CODEOWNERS file to adhere to the Salesforce Open Source project requirements.

I've also added comments to match https://github.com/slackapi/deno-slack-sdk.

Requirements

• I've read and understood the
  Contributing Guidelines
  and have done my best effort to follow them.
• I've read and agree to the
  Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.76%. Comparing base (2c2130c) to head (b05fc1c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #90   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         176      176           
  Lines        9525     9525           
  Branches       10       10           
=======================================
  Hits         9503     9503           
  Misses         22       22           

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[mwbrooks]

@misscoded Thanks a lot! Faster than the GitHub Checks! ⚡