Add Slack message reference parsing utilities

[Cr1stal]

Summary

Adds syntax-only parsing utilities for Slack message references.

Related to #1108.

• Add slack_bolt.message_references for parsing mrkdwn references in message text
• Support user, channel, user group, special mention, date, link, and unknown references
• Add tests for labeled/unlabeled references, unknown references, date links, and escaped or multiline angle brackets
• Document parse_slack_references() in the message-listening guide

Testing

• ./scripts/install_all_and_run_tests.sh
• ./scripts/format.sh --no-install
• ./scripts/lint.sh --no-install
• ./scripts/run_tests.sh tests/slack_bolt/test_message_references.py tests/slack_bolt/util/test_util.py
• ./scripts/run_mypy.sh --no-install
• git diff --check

Category

• 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

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.

[WilliamBergamin]

Hi @Cr1stal thanks for your interest in our project 💯

As mentioned in #1518 we don't have immediate plans to add this feature, this also involves maintaining and ensure its quality over time and as the project evolves, for this reason I've decided to close this PR

If we commit to introducing this feature we can always revisit/reopen these changes 🙏