Add YAML validation workflow

[sanmai]

This workflow will validate .yaml files for general correctness everywhere except for vendored dependencies

• It should also enable running other workflows, since IIRC unless there's a single workflow already no other workflow will run.
• GitHub doesn't do that last time I checked: if a workflow config has a syntax error, workflow does not run.

Testing

The file itself should be valid YAML:

yamllint -d relaxed .github/workflows/validate_yaml.yaml