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.
Built from https://develop.svn.wordpress.org/trunk@61476
git-svn-id: http://core.svn.wordpress.org/trunk@60788 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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