refactor: remove unused .slackignore support and dependencies 💀

[mwbrooks]

Summary

This pull request removes support for the .slackignore file. While the Slack CLI would read existing a .slackignore file, it never used the contents. Long ago, this feature was intended to ignore packaging Deno Project files for deployment.

Going forward, we would offer configurations such as this in the .slack/config.json file.

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 64.84%. Comparing base (158bfa7) to head (d3fa4de).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   64.83%   64.84%   +0.01%     
==========================================
  Files         213      212       -1     
  Lines       17550    17538      -12     
==========================================
- Hits        11379    11373       -6     
+ Misses       5105     5097       -8     
- Partials     1066     1068       +2     

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[zimeg]

@mwbrooks What a nice find! And TIL about this past feature! 🪬 ✨

[zimeg]

🐊 praise: See ya later aligator!