COC Permitir comentários em JSON

The filetype need to be jsonc.
To make Vim treat all json files as jsonc:

```
augroup JsonToJsonc
    autocmd! FileType json set filetype=jsonc
augroup END
```
Or you can use a modeline (:help modeline) at the very top or bottom of an individual json file, to set the filetype to jsonc for only that file:

// vim: ft=jsonc
Rajprakhar