Build/Test Tools: Synchronize svn:ignore with .gitignore.

Adds `svn:ignore` entries for Gutenberg-generated files that were added to `.gitignore` in [61438].

The following entries were added:
* Root directory: `gutenberg`
* `src/wp-includes`: `blocks`, `build`, `class-wp-block-parser-block.php`, `class-wp-block-parser-frame.php`, `theme.json`

Props peterwilsoncc.
See #64393.


git-svn-id: https://develop.svn.wordpress.org/trunk@61476 602fd350-edb4-49c9-b593-d223f7449a82

Author: youknowriad