Enable typescript sourcemaps for debugging locally installed labextensions

[telamonian]

Our webpack config for the release build is currently broken w.r.t. source maps in labextensions. Effectively, what happens right now is that even if an extension ships it's own typescript source maps in perfect working order, when jupyter lab build is run those .js.map files will be thrown out in favor of (javascript, not typescript) source maps produced by the source map tool in webpack.

This is a minimal fix that just covers the case of a locally installed extension. There are other use cases that we should maybe support (normally installed extensions, core packages in a release build), but we should definitely support typescript source maps for labextensions under active development (ie locally installed extensions).

References

This issue has been fixed and was rebroken at least once before. I think the complexity of having two source map tools (one in tsc and one in webpack) in the build chain, each with their own arcane set of options, probably has something to do with it.

refs:
#2922 (original fix)
#3093 (rebreak)
jupyterlab/jupyterlab-git#447 (example of PR to correctly configure ts source maps in an extension)

Code changes

Made needed tweaks to jupyterlab/staging/webpack.config.js and copied them over to dev_mode/webpack.config.js

User-facing changes

Extension devs will get typescript source maps to use when debugging local extensions in eg DevTools, given the correct config . Easiest config is to make sure the following is in tsconfig.json:

{
  "compilerOptions": {
    "inlineSources": true,
    "sourceMap": true,
    ...
  },
  ...
}

Backwards-incompatible changes

None

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