Integrate the coc plugin into vue-preview

[johnsoncodehk]

Hi @yaegassy 👋, now @volar/preview already renamed and move to a new repo (https://github.com/johnsoncodehk/vue-preview), and the preview features becomes a separate vscode extension.

I hope the coc plugin of @volar/preview can be integrated into the new repo, so that future changes to @vue-preview/core can update the coc plugin in time. What do you think?