Editor: Ensure the required assets for the block directory are enqueued.

This fixes an issue where the block directory appears unstyled due to the stylesheets not being enqueued.

Props ryelle, timothyblynjacobs, earnjam.
Fixes #50661.
Built from https://develop.svn.wordpress.org/trunk@48537


git-svn-id: https://core.svn.wordpress.org/trunk@48299 1a063a9b-81f0-0310-95a4-ce76da25c4cd