Add yamllint workflow, validates .yaml files

[sanmai]

This has two effects:

• It will validate .yaml files for general correctness everywhere except vendor and dev-tools/vendor
• And as a side-effect, it might enable other workflows to happen. E.g. we will no longer need to consult with forks to see if Run static code analysis off GitHub Actions #5295 Add GitHub Workflows CI, including testing on PHP 8 and on macOS/Windows/Ubuntu #5268 Migrate to a hybrid of GH actions and Travis #5183 etc are working.

[keradus]

Thank you @sanmai.