Fixes#7408.
For kmc*.cmd, enables source maps. If you are running kmc from node on
your own system, you may need to enable source maps in other ways.
Note that error reports should be going through to Sentry, and source
maps should already be used for those.
Fixes#7237.
Full clean, configure and build will be required for Core, Developer
after this change.
kmc is much less unwieldy a name than kmldmlc, and becomes our platform
for future compiler integration. Start of implementation for #5283 also.
