Add landscape.yaml validation workflow

[tegioz]

This workflow validates that the landscape.yml file is valid on every pull request. It'd be great if you could update the repository's settings so that PRs cannot be merged unless this check passes. This way you'll ensure that the landscape.yml stays valid and the landscape build doesn't break.

[tegioz]

@wrathwolf don't worry about the check not passing at the moment. This was expected because the landscape.yml file contains some errors at the moment. @cynthia-sg will submit a PR shortly to address them and this validation will help you keeping it valid going forward. As soon as Cynthia's PR is merged please mark this check as required 🙂