[Developer] Web-oriented build product sourcemaps

**Is your feature request related to a problem? Please describe.**
When debugging a keyboard in KeymanWeb, there's no clear reference to the original KMN source.

**Describe the solution you'd like**
It would be nice to support JS source-mapping for our compiled Web keyboards. If done right, keyboard developers should be able to set breakpoints in a browser's developer mode on keyboard rules and step through the keyboard's execution.

Of course, there's then the question of how far we'd want to integrate it. Perhaps those would be questions better left to followup work; these are the parts that "require design work."

**Useful references**
- https://sourcemaps.info/spec.html
- Useful package: https://github.com/Rich-Harris/vlq (especially https://github.com/Rich-Harris/vlq/tree/master/sourcemaps)
- handles the compact decoding and encoding used for storing sourcemap data structures