Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

To allow rule configs to have an array with just a severity:

{
    rules: {
        semi: ["error"]
    }
}

What changes did you make? (Give an overview)

• Updated RuleConfig to also match [Severity].
• Updated types tests to verify this change.

Related Issues

Split from eslint/eslint#19843 (comment)

Is there anything you'd like reviewers to focus on?