calypso-build: add webpack config for output.jsonpFunction

[creativecoder]

Changes proposed in this Pull Request

• Add configuration for output.webpackjsonpFunction in the calypso-build package

The webpack docs state

If multiple webpack runtimes (from different compilations) are used on the same webpage, there is a risk of conflicts of on-demand chunks in the global namespace.

This seems to be a source of WordPress plugin conflicts, when multiple plugins use webpack to build js. For example: Automattic/jetpack#17289

Adding this config allows plugins that use calypso-build's webpack config, like Jetpack, to customize the webpack jsonp global variable name and avoid these conflicts.

Note that this config is changing to chunkLoadingGlobal in webpack 5, so we might want to address that in some way, as well.

Testing instructions

• Add output-jsonp-function to a webpack build process that uses getWebpackConfig from calypso-build (Jetpack example: Automattic/jetpack@33815b4)
• Check the output js, and see that window["webpackJsonp"] uses what you specified for the global variable

Required for fixing Automattic/jetpack#17289

[matticbot]

Test live: https://calypso.live/?branch=add/webpack-jsonpfunction-config

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[jsnajdr]

Nice! 👍

[jsnajdr]

Maybe also add a line to the docblock comment few lines above? The existing documentation there is not terribly helpful though 🙁 (output-path sets output path and output-library-target sets output library target, who would have guessed that)

[creativecoder]

👍 Done in b42eaaa

[creativecoder]

@scinos @jsnajdr Thanks for the prompt review!

How do you recommend getting the calypso-build package updated so this change can be used externally? Okay to change package version in this PR (to 6.2.1) and update changelog.md, or should that be handled separately?

[jsnajdr]

@creativecoder We usually do the release chores (changelog, version bump) as a separate PR. There is a P2 post about how to publish packages to NPM (with a video!) here: p4TIVU-9mL-p2. The NPM publishing docs in the Calypso repo also look reasonably up to date.