Improvement related to #199: add event type name validation

[seratch]

This pull request improves the event listener registration in the aim of helping developers easily understand what's wrong with message.* values for event types. ref #199

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #201 (83ec7b8) into main (90bfe6a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   91.60%   91.61%   +0.01%     
==========================================
  Files         159      159              
  Lines        4750     4760      +10     
==========================================
+ Hits         4351     4361      +10     
  Misses        399      399              

Impacted Files	Coverage Δ
slack_bolt/listener_matcher/builtins.py	93.83% <100.00%> (+0.24%)	⬆️
slack_bolt/logger/messages.py	88.23% <100.00%> (+0.48%)	⬆️

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[seratch]

@aoberoi @stevengill @mwbrooks @misscoded
We may want to add the same validation to Bolt for JS and Bolt for Java. If there is no objection to it, I am happy to work on it.

[seratch]

As we do not have enough time until v1.2.0 release, I will be waiting for feedback for a few more days. If this is fine, I will merge this change for v1.2.1.

[seratch]

This pull request is ready to merge. I will wait for responses from other maintainers for a few more business days before merging it. For feature parity across SDKs, I started a discussion at slackapi/bolt-js#757