ci: implement a PR title linter

[alessfg]

Proposed changes

This PR adds a GitHub Action workflow to lint PR titles and ensure they follow the Conventional Commits spec. This accomplishes two things:

1. Streamlines our commit history, making it easier to grep the contents/focus of any given commit/PR.
2. Streamlines our release process (still a WIP) by making it very easy to categorize changes within our release notes.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• [x] I have read the contributing guidelines.
• [x] I have signed the F5 Contributor License Agreement (CLA).
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works.
• [x] If applicable, I have checked that any relevant tests pass after adding my changes.
• [x] I have updated any relevant documentation (e.g. README.md).