Build: Add globalThis DefinePlugin config to webpack

[sirreal]

This updates the build to account for the following PR:

WordPress/gutenberg#61486

This affects the build of JavaScript packages to ensure dead-code elimination works correctly.

It should be harmless to land this PR before packages are updated. The older configuration can be removed after the packages are updated.

Trac ticket: https://core.trac.wordpress.org/ticket/61262

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[swissspidy]

I‘m confused, why does this require a core change?

Does this mean any other projects using these packages also need to update their configs?

I thought those variables in the GB packages are replaced during compile time for dead code elimination and shouldn‘t be present in the distributed packages.

[youknowriad]

Does this mean any other projects using these packages also need to update their configs?

I think it's not mandatory and these values would act as "false" by default but if you want to tree-shake them out of the code base, it's better to update the config.

[youknowriad]

The variables are present in the packages (just like the previous ones were) to allow the folks importing the packages (Gutenberg or Core) to decide whether to enable something or not (disabled by default)

[sirreal]

The variables are replaced when they're compiled as Core Scripts or Script Modules. They're shipped in the npm packages (that has not changed at all with WordPress/gutenberg#61486).

Gutenberg does that here:

https://github.com/WordPress/gutenberg/blob/f49e0b5958adad9fbd211d5ded07b4cabf5bfec3/tools/webpack/shared.js#L65-L76

This PR is updating Core's build configuration accordingly.

[sirreal]

Does this mean any other projects using these packages also need to update their configs?

Yes and no. If folks are actually compiling projects with the npm packages (bundled, not provided by WordPress) and had configured their bundler to perform replacements then they'd need to update. I believe those are very exceptional conditions.

There shouldn't be anything harmful about this change for most projects, in fact it's safer as it should remove some of the problems that process.env has.

[youknowriad]

LGTM

[gziolo]

Committed with https://core.trac.wordpress.org/changeset/58193. We will have to follow up with the removal of old replacements when the next version of WP packages gets published to npm.

[sirreal]

#6904 is the follow-up to remove the legacy configuration.