Sync Gutenberg reusable blocks (`wp_block` post type)

Gutenberg stores re-usable blocks in `wp_block` post type.

We should add these to sync whitelist;

https://github.com/Automattic/jetpack/blob/4a8107b7ebd44a38da1e8604e4a0ebe865ea9c7d/sync/class.jetpack-sync-functions.php#L127-L130

See also https://github.com/Automattic/jetpack/issues/9909

Start with sync/class.jetpack-sync-functions.php at the linked whitelist section and inspect how post types are listed for synchronization. Add wp_block to the whitelist, then verify that reusable Gutenberg blocks are included in sync behavior; issue 9909 provides related context.

Written by the indexing model from the issue text.