Add a built-in ruleset to validate Spectral ruleset

[dolmen]

User story.
As a user, I would like to check that the ruleset I've written to use Spectral is valid.

Is your feature request related to a problem?
As Spectral is a JSON/YAML validator and Spectral rulesets are written in JSON or YAML I would like to use Spectral for that task.

Describe the solution you'd like
Add a built-in ruleset for Spectral ruleset like we have for OpenAPI and AsyncAPI.

Additional context

• As with v6.0.0 writing a ruleset is mandatory in the CLI arguments, validating a Spectral ruleset is helpful for all users.
• Eat your own dog food

[P0lip]

Heyo!
I agree. We probably wouldn't expose a ruleset, but a CLI command for that, i.e. spectral ruleset validate or alike.
This would probably output errors in a similar way to spectral lint.
I have some code somewhere that adds it, but haven't had enough time to get this over the finish line.