Add YAML schema validation to CI process

[neilenns]

This PR adds two YAML validation steps to the existing CI process. This should guard against issues like the one introduced in #17.

• Adds schema files for airports.yml and navaids.yml
• Adds verification steps to the existing workflow, before the JSON files are generated
• Updates to actions/checkout@v4 to remove a warning since I was editing the workflow anyway

Here's an example of a failed run against the file checked in with #17: https://github.com/neilenns/navdata/actions/runs/10392918979/job/28779239721

Here's an example of a successful run against the current checked in yaml: https://github.com/neilenns/navdata/actions/runs/10392937229

The job is named "Validate and generate" and can be added as a required status check in GitHub's branch protection rules to guard against merges that fail validation.