Allow custom webpack config for federated extensions

[jtpio]

References

Fixes #9175

Should also cover the use case mentioned in #4328

Code changes

Allow extension authors to provide a custom webpack config for their federated extension.

TODO

• Read default webpack.config.js from the federated extension
• Add a subkey in the jupyterlab config in the package.json to point to the custom webpack config
• Add documentation in the extension developer guide: mention that this extension point is experimental

User-facing changes

None

Backwards-incompatible changes

None user facing.

For developers, this exposes a direct way to configure webpack for federated extensions.

[jupyterlab-dev-mode]

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[jtpio]

See #9175 for more context.

And jtpio/jupyterlab-wasm-example#4 as an example for what it would look like from a third-party extension.

[jtpio]

This makes it opt-in, meaning that even if a webpack.config.js file is at the root of the federated extension folder, it will not be picked up unless explicitly specified in package.json under the jupyterlab.webpackConfig subkey.

[jasongrout]

Thanks. I think that is really important to be explicit about this, especially given it is an experimental feature. We only want it available to people that really explicitly want to use it.

[blink1073]

LGTM, thanks!

[jasongrout]

Thanks!