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.