ci: Add semantic PR title validation

[barbeau]

Summary:

This enables a bot to enforce semantic PR titles, same as static validator - see https://github.com/MobilityData/gtfs-validator/pull/497 and https://github.com/MobilityData/gtfs-validator/blob/master/.github/semantic.yml.

Expected behavior:

Semantic bot should run on pull requests

Please make sure these boxes are checked before submitting your pull request - thanks!

• [ ] Run the unit tests with mvn test to make sure you didn't break anything
• [ ] Format the title like "Fix # - " (for example - "Fix #1111 - Check for null value before using field")
• [ ] Linked all relevant issues
• [ ] Include screenshot(s) showing how this pull request works and fixes the issue(s)