Block Editor: Add an `is_block_editor()` method to `WP_Screen`.

This method allows checking (or setting) whether the block editor is loading on the current screen.

Merges [43777] from the 5.0 branch to trunk.

See #45037.

Built from https://develop.svn.wordpress.org/trunk@44130


git-svn-id: https://core.svn.wordpress.org/trunk@43960 1a063a9b-81f0-0310-95a4-ce76da25c4cd