Build: Introduce SCRIPT_DEBUG global in webpack processing

[gziolo]

Backports the same changes to the webpack config in the Gutenberg plugin with ​https://github.com/WordPress/gutenberg/pull/50122.

The warning from @wordpress/warning no longer worked correctly with webpack 5. In practice, it no longer called console.warn. To fix it, the usage of process.env.NODE_ENV check got replaced with another optional global: SCRIPT_DEBUG. All the tools used in the Gutenberg, get updated to work with this new constant, including @wordpress/scripts. This way, developers are able to guard code that should be run only in development mode. In WordPress core, the same constant needs to be added mostly to ensure that the code behind the check gets completely removed in production mode.

It's an enhancement on top of #59411 and can be tested after WordPress packages get backported from the Gutenberg plugin.

[gziolo]

@youknowriad, do you know if we can remove process.env.FORCE_REDUCED_MOTION? I don't see it in the Gutenberg plugin anymore:

​https://github.com/WordPress/gutenberg/blob/trunk/tools/webpack/shared.js#L65-L74

Trac ticket: https://core.trac.wordpress.org/ticket/59407

I can confirm that webpack still builds correctly and the block editor loads as expected.

It looks like process.env.FORCE_REDUCED_MOTION is no longer used with WordPress packages, so I'm going to remove that global. It got removed with this commit: ​https://github.com/WordPress/gutenberg/commit/914f543e1e50d8c65c05a417efbdf539ef0d7a63.

[gziolo]

In 56699:

Build: Introduce SCRIPT_DEBUG global in webpack processing

Backports the same changes to the webpack config in the Gutenberg plugin with ​https://github.com/WordPress/gutenberg/pull/50122.

The warning function from @wordpress/warning no longer worked correctly with webpack 5. In practice, it no longer called console.warn. To fix it, the usage of process.env.NODE_ENV check got replaced with another optional global: SCRIPT_DEBUG. All the tools used in the Gutenberg, get updated to work with this new constant, including @wordpress/scripts. This way, developers are able to guard code that should be run only in development mode. In WordPress core, the same constant needs to be added mostly to ensure that the code behind the check gets completely removed in production mode.

Fixes #59407.

Committed with https://core.trac.wordpress.org/changeset/56699.

[gziolo]

I can confirm that warning messages are correctly removed from production builds in WordPress 6.4 Beta 1.