Create sourcemaps for the build

[zpao]

Browserify sort of allows you to do this, but it puts them inline in the same file, which is far from ideal. We should consider working with the browserify team and make it possible to build the map outside of the bundle or do something else (might be able to extract sourcemap out, might have to stop using browserify)

[andreypopp]

https://github.com/thlorenz/mold-source-map can be used to extract source maps from a bundle
and related browserify issue — browserify/browserify#339