feat(types): add markdown block

[zimeg]

Summary

This PR adds the markdown block to @slack/types.

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 92.71%. Comparing base (5bcef49) to head (5df47d2).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2296   +/-   ##
=======================================
  Coverage   92.71%   92.71%           
=======================================
  Files          38       38           
  Lines       10607    10607           
  Branches      683      683           
=======================================
  Hits         9834     9834           
  Misses        761      761           
  Partials       12       12           

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	94.74% <ø> (ø)
oauth	77.39% <ø> (ø)
socket-mode	61.87% <ø> (ø)
web-api	97.97% <ø> (ø)
webhook	96.66% <ø> (ø)

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[zimeg]

📝 Testing notes for the kind reviewers can use changes to the sample of slack-samples/bolt-js-assistant-template#47 and this PR:

$ npm pack  # Package these changes before testing the sample app
$ cd
$ slack create asdf -t slack-samples/bolt-js-assistant-template -b zimeg-feat-markdown-block
$ cd asdf
$ npm run check                                         # Attempt the type check test
$ npm install ../path/to/node-slack-sdk/packages/types  # Install these changes
$ npm run check                                         # Confirm the sample passes the type check

[WilliamBergamin]

Nice 💯

[WilliamBergamin]

Alphabetical 🙏

[zimeg]

I was not sure where else to add a new block! 😉

[zimeg]

@WilliamBergamin Thanks for the fast review! I'll merge this now but will plan for a soonish release 🚢 💨

[pedropaccola]

Please do release it!

[zimeg]

@pedropaccola Thanks for the bump - we'll aim to release this after #2330 lands. In this meantime I might recommend using regular JSON for the block:

{
  "type": "markdown",
  "text": "**Lots of information here!!**",
}