Refactor diff.ts and format.ts Code into a Generic npm Package

[rindeal]

Hi @int128,

First, thank you for your work on this repository.

I am writing to ask if you would consider refactoring the code, particularly the diff.ts and format.ts files, into a generic npm package, such as "git-diff-to-gha-md" or similar. This would allow the functionality to be reused in other GitHub Actions, such as one that writes to GITHUB_STEP_SUMMARY instead of pull request comments.

This enhancement would greatly benefit users who need similar functionality but in different contexts. I believe it would make your work even more versatile and widely adopted.

Thank you for considering this request. I look forward to your response.

Best regards