This extension have been moved into the Roc extension monorepo here.
January 2, 2018 ยท View on GitHub
This change has been made to make it easier to work with the extensions overall and release new versions quickly. Please use this repo for reporting issues and open pull requests.
The code for roc-plugin-style-less can be found here.
roc-plugin-style-less 
Plugin providing Less support for Webpack
To be used together with Roc.