Support sourcemap #12
Description
We have to implement environment.getSourcemapGenerator. The default implementation in less-node (less-browser doesn't support sourcemap) also depends on source-map:
https://github.com/less/less.js/blob/3f05b5cde6803064bef0ddc837ec1060c9e83635/packages/less/src/less-node/environment.js#L13-L15
Then create less instance via createFromEnvironment:
https://github.com/less/less.js/blob/3f05b5cde6803064bef0ddc837ec1060c9e83635/packages/less/src/less-node/index.js#L5
Maybe we can map source-map to a global variable so that we can mock it with a simpler implementation?