Add code lens diagnostics

[Andrew Stewart]

I've seen some other coc extensions use the code lens feature to put some nice annotations in buffers. How doable would it be to make coc-eslint display diagnostics inline with code lens similar to https://marketplace.visualstudio.com/items?itemName=usernamehw.errorlens ?

[Qiming zhao]

There's only virtual text support for diagnostics :h coc-config-diagnostic