Meta Boxes: Add `__back_compat_meta_box` and...

Meta Boxes: Add `__back_compat_meta_box` and `__block_editor_compatible_meta_box` flags to meta boxes.

When meta boxes are registered, they can use the `__back_compat_meta_box` and `__block_editor_compatible_meta_box` flags, to show whether this registration just exists for if the classic editor is loaded, and whether this meta box is compatible with the block editor.

When a meta box marks itself as incompatible with the block editor, and `WP_DEBUG` is enabled, a warning will show inside that meta box in the classic editor.

As all core meta boxes have been recreated in the block editor, they can be marked with the `__back_compat_meta_box` flag.

Merges [43779] from the 5.0 branch to trunk.

See #45112.


Built from https://develop.svn.wordpress.org/trunk@44132


git-svn-id: https://core.svn.wordpress.org/trunk@43962 1a063a9b-81f0-0310-95a4-ce76da25c4cd