Integrate sourcemap support to enable features like compiling javascript with webpack and minification

Requires source map support/information back from SAM CLI before proceeding.

Acceptance criteria:
* nodejs projects using webpack can run locally
* nodejs projects using webpack can be debugged locally (hitting breakpoints, etc)

Begin by checking whether SAM CLI now returns the source-map information this work requires, then investigate the toolkit's local webpack execution and debugging entry points. Done means Node.js projects using webpack run locally and support local debugging with working breakpoints, as stated in the acceptance criteria.

Written by the indexing model from the issue text.