Build: Add new Webpack plugin to handle library default export

[gziolo]

Description

Closes #6748.

Previously: #6584 (comment)

The intention with consuming from the WordPress npm organization packages is that each module would have itself exposed as an equivalent camel-case form as a property on window.wp. This already works well for most packages, allowing developers to use e.g. wp.i18n.__ (the __ named export from @wordpress/i18n).

However, for packages which have a default export, the usage is not as expected:

gutenberg/lib/client-assets.php

Line 1032 in 73f2331

wp.domReady.default( function() {

Note the .default, which ideally should be omitted, calling instead as wp.domReady( fn ).

I played with the different approaches proposed in #6748 and ended up implementing a custom Webpack plugin, which works very similar to what libraryExport: 'default' does as explained in Webpack docs:

libraryExport: "default" - The default export of your entry point will be assigned to the library target:

// if your entry has a default export of `MyDefaultModule`
var MyDefaultModule = _entry_return_.default;

Plugin's implementation is based on the ExportPropertyMainTemplatePlugin core plugin. The only difference in here is that instead of applying this transformation to all entry points, we will be able to provide the list of chunks where this should be applied:

new LibraryExportDefaultPlugin( [ 'dom-ready' ].map( camelCaseDash ) )

How has this been tested?

Make sure npm run dev and npm run build work properly.

It should be possible to execute the following code on the JS console when running Gutenberg:

wp.domReady( () => {} );

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[aduth]

Looks (and works) great 👍

Unfortunate that we can't detect this automatically. Guessing that's impossible / prohibitively difficult to implement. At least for the time being, I'm more concerned with avoiding developers coming to expect the .default property to exist.