Support for GitHub Merge Queue (merge_group Event)

Summary

This pull request adds support for GitHub's merge queue functionality by handling the merge_group event in the commitlint-github-action. It enables users who utilize GitHub's merge queue feature to lint commit messages effectively during the merge process.
Resolve #744 #757

Changes

1. Update action.mjs and tests to Support merge_group Event:

• Added logic to detect when the action is triggered by a merge_group event.
• Updated the way commits are retrieved in this context to ensure the correct commits are linted.
• Verified that the correct commits are being linted in this context.

2. Update README

• Added a new section on using the action with GitHub Merge Queues:
  • Provided examples of workflows that listen to the merge_group event.
  • Explained necessary configurations and considerations when using the merge queue.

Testing

I've tested the updated action extensively to ensure it works correctly with the merge_group event.

Test Repository: yossi-test-org/test-merge-queue
Test Pull Request: PR #24

Test Steps

1. Set Up Merge Queue:

• Enabled the merge queue feature in the test repository.
• Configured branch protection rules to require the commitlint status check.

2. Created Workflows:

• Added a workflow that listens to the merge_group event and runs the commitlint action.
• Added a placeholder workflow for the pull_request event with a job named commitlint to satisfy merge queue requirements.

3. Tested Pull Requests:

• Opened pull request with various commit messages to test linting behavior.
• Added pull request to the merge queue to trigger the merge_group event.

4. Verified Action Execution:

• Confirmed that the commitlint action runs successfully during the merge_group event on the squashed commit.
• Checked that the action lints the correct commit and reports any issues appropriately.

For testing I also pushed the image to a container registry and ran the workflow against it (https://github.com/users/YossiSaadi/packages/container/package/commitlint-github-action)