Block Editor: Add helper functions for displaying the editor.

`use_block_editor_for_post()` and `use_block_editor_for_post_type()` determine if the block editor should be loaded.

`get_block_categories()` and `get_block_editor_server_block_settings()` provide data be included while the block editor is loading.

Props pento.

Merges [43762] to trunk.

See #45110.
Built from https://develop.svn.wordpress.org/trunk@44122


git-svn-id: https://core.svn.wordpress.org/trunk@43952 1a063a9b-81f0-0310-95a4-ce76da25c4cd