Add sourcemap generation to grunt

Following on from MDL-51582, it would be nice to add a new grunt css-dev task which doesn't run by default and generates sourcemaps.

I tried this a few times but I couldn't get the right combination of sourceMapBasepath/sourceMapRootpath/ combos so that the sourcemap actually pointed to the correct url for the less files. My setup was moodle served on moodle.dev/pm/