Rename GUTENBERG_PHASE to IS_GUTENBERG_PLUGIN

[noisysocks]

Gutenberg currently has an environment variable at process.env.GUTENBERG_PHASE.

It is set to 1 in Core:

https://github.com/WordPress/wordpress-develop/blob/a9e66f4bcb8a857c78ac29f7de30f8f9f3d129c5/tools/webpack/blocks.js#L187

It is set to 2 in the Gutenberg plugin:

https://github.com/WordPress/gutenberg/blob/trunk/package.json#L22

The original intention with this variable was that GUTENBERG_PHASE would be incremented gradually. 2 for full site editing, 3 for collaboration, 4 for multilingual support, etc. This would then be used for feature gating so that we're not shipping experimental features to Core.

In practice, this variable has only ever been used as a way to mark code which should be stripped out of the JavaScript when bundled for use in Core.

I propose we give the variable a new name that explains how it's used in practice. IS_GUTENBERG_PLUGIN is my pick as it aligns with a PHP constant recently introduced in #37021.

cc. @WordPress/gutenberg-core

[youknowriad]

That makes sense 👍