Log message:
Add action-validator: Validate GitHub Action and Workflow YAML files

action-validator is a standalone tool designed to "lint" the YAML files
used to define GitHub Actions and Workflows. It ensures that they are
well-formed, by checking them against published JSON schemas, and it
makes sure that any globs used in paths/paths-ignore match at least one
file in the repo.

The intended use case for action-validator is in Git pre-commit hooks
and similar situations.