This changeset improves the Gutenberg build integration to simplify the developer workflow and reinstore a flow similar to how package dependencies worked before the Gutenberg checkout-and-build approach was introduced.
Key improvements:
* Automatic rebuild on ref change: Adds a new `gutenberg:sync` script that stores a hash of the built ref in `.gutenberg-hash` and only rebuilds when the ref changes.
* Full integration on `npm install`: Running `npm install` now produces a fully working development environment with Gutenberg assets in `src/`.
* Clean Gutenberg checkout: Restores Gutenberg's `package.json` after the build completes.
* Stops copying `.js.map` files to `wp-includes/js/dist` since they reference non-existent paths.
* Remove package.json files from the build folder.
* Avoid closures and use prefixed functions.
* Updates build checks to use `jquery.js` instead of `edit-post.js` as the build indicator.
Props youknowriad, ellatrix, mcsf, dmsnell, ntsekouras, jorgefilipecosta, tobiasbg, peterwilsoncc.
Fixes#64393.
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