Add allowComments Support

Is your enhancement related to a problem? Please describe.

Sometimes comments aren’t supported. I.e. a tool might strip comments.

Describe the solution you would like

From the language service it probably makes sense to add a new option for doValidation.

interface LanguageService {
  doValidation(document: TextDocument, isKubernetes: boolean, allowComments: boolean): Promise<Diagnostic[]>
}

If this setting is true, any comment nodes will be reported.

For the language server it makes sense to add an option yaml.allowComments.

Describe alternatives you have considered

doValidation could accept an object with named properties instead.

interface DoValidationOptions {
  allowComments?: boolean;
  isKubernetes?: boolean;
}

interface LanguageService {
  doValidation(document: TextDocument, options?: DoValidationOptions): Promise<Diagnostic[]>
}

Instead of reporting every comment node, only the first or last comment in a group of comments will be reported.

# This comment will be reported
# This comment will not be reported
# This comment will not be reported

foo: bar

# This comment will be reported
# This comment will not be reported
# This comment will not be reported

Or only one comment could be reported

# This comment will be reported
# This comment will not be reported
# This comment will not be reported

foo: bar

# This comment will not be reported
# This comment will not be reported
# This comment will not be reported

Additional context

This is a downstream request from remcohaszing/monaco-yaml#28

Start with the LanguageService.doValidation interface and the language server’s yaml.allowComments option described in the issue. Review the downstream request at remcohaszing/monaco-yaml#28 and resolve whether validation takes positional or named options and how comment groups are reported. Done means the option is exposed and comment diagnostics follow an agreed grouping behavior.