What?

This PR allows users of the Dependency Extraction Webpack Plugin "do a simple thing easy" (and complex things still possible) and exclude packages from externalizing in a simple way. It also upstream WooCommerceDEWP feature (woocommerce/woocommerce-admin#7034). Or, in other words, publish the private feature of BUNDLED_PACKAGES.

It also includes a tiny clarification in the README (e8f3e85). The example was using the entrypoint for the assets file, whereas the plugin uses output config for that. (fixes #49872)

Why?

1. To allow doing a simple thing easy, as in the majority of DEWP's use-cases, I've seen requestToExternal is used to filter a list of packages.
2. To reduce the confusion of a non-Woo specific (at least conceptually) behavior of providing a simple list of packages to filter, not basing available in WP version of DEWP.
3. To reduce the confusion of WooCommerceDependencyExtractionWebpackPlugin's bundledPackages not working for @wordpress/ packages.

How?

This PR changes the DEWP constructor to take excludedExternals option to use it to filter packages processed in externalizeWpDeps before any requestToExternal is called.
Turns the old BUNDLED_PACKAGES to the option's default value and exposes it as a static property to allow extending it by consumers.

Testing Instructions

Run automated tests

1. npx wp-scripts test-unit-js --config test/unit/jest.config.js packages/dependency-extraction-webpack-plugin/

Manual testing

1. Create a project that uses @wordpress/components, @wordpress/url, @wordpress/icons and @wordpress/interface
2. Setup webpack config to use

new DependencyExtractionWebpackPlugin( {
	requestToExternal( package ) { console.log( package ) },
	excludedExternals: [
		...DependencyExtractionWebpackPlugin.excludedExternals,
		'@wordpress/url'
	],
} ),

3. Confirm that @wordpress/url, @wordpress/icons and @wordpress/interface gets bundled, and none of them is logged.

Screenshots or screencast

Additional info:

1. I was considering keeping the bundledPackages name. This would keep the old name, and make it directly compatible with existing WooCommerce implementation. However:
   1. IMO, it's not the accurate name, as those packages may not get bundled at all. For example, if none of them is actually used & imported.
   2. The behavior is slightly different than in Woo, as the package will not reach options.requestToExternal, so this way we avoid breaking change.
   3. We can still have a direct migration path or alias for the old name.
2. I was trying to write tests to check that the default exclusions /icons and /interface are bundled, but didn't manage for test build to process it. I guess it's related to the monorepo setup. For lodash it works as expected.
3. Initially I added it explicitly as

    static excludedExternals = // ...

   but eslint fails to parse it. Probably due to the old ES version used.

cc @gziolo as you reviewed my latest DEWP related issues
cc @louwie17 as someone who implemented it in Woo, and could give a context about why it eventually landed as bundledPackages not excludedExternals.