Add option to specify schema validation severity level

Is your enhancement related to a problem? Please describe.

I am using monaco-editor and monaco-yaml. I need to configure the editor such that user input YAML is validated against a schema, with validation failures reported as errors to the user (as opposed to warnings).

Describe the solution you would like

The built-in JSON support of monaco-editor provides a configurable severity for schema validation failures (see here). I would suggest similar functionality for yaml-language-server. If this option is added, the monaco-yaml maintainer has expressed support for passing the option through: remcohaszing/monaco-yaml#149

Describe alternatives you have considered

@remcohaszing and I also discussed adding a hook into monaco-yaml to allow post-processing of warnings/errors, but this approach is more consistent with monaco-editor's built-in JSON support.

Start by comparing yaml-language-server's schema validation configuration with the linked Monaco JSON API, then trace how the option could be exposed to monaco-yaml via remcohaszing/monaco-yaml#149. Done means schema validation severity can be configured and passed through consistently, with the relevant behavior covered by the project's existing tests.