feat(slack-cli): add slack-cli package and enable on work MacBook

Add Python-based slack-cli package that reads Slack messages from local
Chromium IndexedDB cache. Includes pinned dependencies for dfindexeddb,
python-snappy, and zstd. Enable on mac-va-mbp-work home config.

homes/aarch64-darwin/evanreichard@mac-va-mbp-work/default.nix

@@ -57,6 +57,7 @@ in
     # colima
     docker
     keycastr
+    reichard.slack-cli
     _1password-cli
   ];
 }