fix(yaml): add missing formatting provider

[KermanX]

ls.doFormat has another param with type CustomFormatterOptions, in this PR this is unused - I am not 100% sure about this.

[remcohaszing]

Thanks for your contribution. We decided to not implement yaml-panguage-servers formatting API, because it’s just a poor wrapper around Prettier 2. If you want to format YAML code with Prettier, you can better use volar-service-prettier.