missing virtualTexthl breaks lcn

[circld]

This is a minor change to add highlight groups in the LanguageClient-neovim configuration. The absence of these was leading to the various language servers to exit silently (or maybe not even starting), effectively breaking the plugin for me. Adding these four lines fixed 🤷

Let me know if there's anything else you need from me in order to merge. 👍

[liuchengxu]

Thank you!