Block Editor: Expose value of `user_can_richedit()`.

In order for the Block Editor to know whether to allow visual editing, it needs to know the value of `user_can_richedit()`.

Props youknowriad, danielbachhuber.

Merges [43916] into trunk.

Fixes #45375.
Built from https://develop.svn.wordpress.org/trunk@44267


git-svn-id: https://core.svn.wordpress.org/trunk@44097 1a063a9b-81f0-0310-95a4-ce76da25c4cd