[dev] display source mapped line numbers

@xtuc has a WIP PR that would add sourcemaps to anything published with wrangler. we should utilize these maps in wrangler dev. @xtuc has already implemented source maps in the devtools protocol - the source map is accessible as base64 encoded json

Possible implementations

• Implement in the runtime
  • JavaScript implementation
    • probably easier
  • C++/KJ implementation
    • possibly more performant

@xtuc would need to talk with runtime team about how we would possibly move forward with this approach. He noted a possible concern with regards to leaking meaningful stack traces in response bodies.

• Implement everything in Wrangler
  • could parse base64 sourcemap from devtools and compute accurate line numbers from lineNumber and columnNumber fields from devtools
  • alternatively could read in source map from webpack config and not worry about base64 and compute accurate line numbers from lineNumber and columnNumber fields from devtools protocol

This approach is probably the easiest but would require any new clients to also implement their own source map handling approach. My guess is that long term we probably want the runtime to support this.

[mroderick]

I think stale bot might have been a bit too keen to close this issue. I would still very much like to have source maps for my worker scripts

[xtuc]

In wrangler dev we should make webpack always generate a sourcemap, serve it with a new local HTTP server and inject a sourceMappingURL in the worker that points locally.

When the worker runtime will load the worker it will send that source map URL to the devtools that it can download from the user endpoint.