Abstract away and document advanced constants used in webpack config

[gziolo]

The documentation around webpack config has been documented in-depth as explained in #8982 (comment).

The last part left is how one can take advantage of global variables to do some advanced work:

• WP_BUNDLE_ANALYZER
• WP_DEVTOOL
• WP_LIVE_RELOAD_PORT

Ideally, those constants should be abstracted with CLI arguments similar to @draganescu did in #14129. In effect, we would be able to run the following, e.g.:

$ npx wp-scripts build --webpack-live-reload-port=12345 --webpack-bundle-analyzer --webpack-devtool=inline-source-map

Example of the existing implementation of similar CLI args for npx wp-scripts test-e2e:
https://github.com/WordPress/gutenberg/blob/master/packages/scripts/scripts/test-e2e.js#L46-L65

[gziolo]

By the way, it also reminds me that we forgot to document custom CLI args passed to wp-scripts e2e - it would be nice to include them as well. Filed as #14494.

[Jackie6]

Hi @gziolo I have created a pull request to update the documentation of wp scripts.

[gziolo]

This issue is waiting for volunteers again.

[gziolo]

A similar issue was raised on WordPress.org blog:

https://make.wordpress.org/core/2020/02/28/new-wordpress-create-block-package-for-block-scaffolding/#comment-38068

[gziolo]

WP_LIVE_RELOAD_PORT won't be promoted to a CLI flag because its future is uncertain. It might get replaces with React Fast Refresh as proposed in #21008.