Build/Test Tools: Update svn:ignore for Gutenberg build artifacts.

Adds missing `svn:ignore` entries for files generated by the Gutenberg build process:
* Root directory: `.gutenberg-hash`
* `src/wp-includes/assets`: ignore all generated files
* `src/wp-includes/blocks`: remove versioned directories and ignore all generated files

Follow-up to [61476].

Props ellatrix.
See #64393.


git-svn-id: https://develop.svn.wordpress.org/trunk@61701 602fd350-edb4-49c9-b593-d223f7449a82

Author: youknowriad