Implement case-insensitive matching for head branch name

[datpmt]

Description:

I want to automatically add the refactor label to a pull request when the head branch name contains the word refactor in any case (e.g., refactor, Refactor, REFACTOR, etc.). This should be a case-insensitive match to ensure flexibility and consistency in labeling.

TEST:

# .github/labeler.yml

refactor:
  - head-branch: 'refactor'

Related issue: Add link to the related issue.

Check list:

• [ ] Mark if documentation changes are required.
• [x] Mark if tests were added or updated to cover the changes.