Contribute to yaml language server with a custom language.

[angelozerr]

In vscode-quarkus we override the YAML language to support Qute support (completion, validaton, etc) in YAML files.

The bad thing is that we loose your YAML support like YAML validation.

We had the same problem with HTML language server and a long time ago I created a PR to contribute to HTML language server with a custom language (qute-html), see PR at microsoft/vscode#146731

It would be anwesome, if vscode-yaml could provide this same feature than HTML.