Build/Test Tools: Support testing unmerged changes from Gutenberg.
The gutenberg.sha property in the package.json file expects a full-length SHA value to be specified in order to download the built assets from the Gutenberg repository and include the files through the build script.
After https://github.com/WordPress/gutenberg/pull/78211, assets are now published for pull_request events in addition to push so long as the HEAD branch of the pull request exists in the repository and not in a fork. This makes the built assets for pull requests available for testing within wordpress-develop before they are merged through a pr-### tag on the GHCR package.
To test a given pull request, just set the gutenberg.sha value to the corresponding pr-### tag. In addition to pull request-specific tags, each wp/X.Y, release/X.Y, and trunk now have a tag that can be used to test the latest changes. Each time build or build:dev is called the script will attempt to confirm the latest version of the assets are present locally, pulling down the latest changes when an update is available.
All of the tags described in this changeset are mutable, so they should never be committed. Only immutable SHA values should be used for gutenberg.sha in commits to this repository.
This commit also adds the related JavaScript files to the TypeScript configuration file.
Props westonruter, jorbin, manhar.
Fixes #65224.