Copy learnings from MobileFrontend's Webpack configuration. If nothing
else, the files are more consistent and easier to diff. When the change
to rename source maps is excluded, the build products are identical.
The following changes were copied:
- DRY up the output directory and source map extension as variables. For
the latter, rename the source map from ".json" to ".map.json". Without
renaming, the build products are unchanged.
- Reduce verbosity. Only report warnings and errors.
- Fail to build when an error occurs.
- Update ordering and add comments for easier reasoning and diffing with
MobileFrontend.
Bug: T212527
Change-Id: Icf11dff91358ad021932aa209c65ed8aac77d12b
