Block Editor: Add extra body classes when the block editor is loaded.

Merges [43780] from the 5.0 branch to trunk.

See #45037.


Built from https://develop.svn.wordpress.org/trunk@44133


git-svn-id: https://core.svn.wordpress.org/trunk@43963 1a063a9b-81f0-0310-95a4-ce76da25c4cd