Implement YAML language service

[davelopez]

There is an existing YAML language service embedded in Red Hat's yaml-language-server. Unfortunately is not like the vscode-json-languageservice so it is not published independently of the server implementation and kind of tailored for Kubernetes and integrated telemetry. Also, it relies on JSON schema for validation, auto-completion, etc.

It will probably be a bigger project, but I think we should try to get inspiration from it and create a more lightweight language service for YAML without the telemetry and other stuff and, at the same time, have the flexibility to directly support the gxformat2 schema based on schema salad

[davelopez]

With #50 and #52 this can be considered done.