Add possibility to validate Helm Charts

[olensmar]

As requested / discussed at https://www.reddit.com/r/kubernetes/comments/zqu7ja/open_source_cli_for_kubernetes_yaml_validation_in/

Helm Lint seems to do some validation - but it's unclear from the documentation what it actually validates - https://helm.sh/docs/helm/helm_lint/

At least we should be able to identify the following:

• invalid value references in templates
• invalid value types in values files (if a schema exists)
• "bad practices" in Chart.yaml
• ?