Created by: sbaechler
This MR provides an opt-in for creating CSS sourcemaps in dev mode. The related issue is #5707 (closed).
When the environment variable DEV_CSS_SOURCEMAPS is set to true, the sourcemaps are generated in development mode as well. Production mode is not affected.
Tested by running 'yarn start' with and without environment variables.
This is the default:
This is with DEV_CSS_SOURCEMAPS=true:
