Add GitHub Action to block unconventional commits

[vitormattos]

Reference:

• https://github.com/LibreSign/libresign/blob/main/.github/workflows/block-unconventional-commits.yml
• https://www.conventionalcommits.org/en/v1.0.0-beta.2/

This is a good first issue for someone who wants to make their first contribution. Feel free to ask questions in the comments if you need help!