Administration: Improve the accuracy of is_block_editor().

Currently, there are a number of scenarios where `is_block_editor()` (and `WP_Screen::is_block_editor`) would incorrectly indicate block editor support at different points of the loading process. Most notably, checking `is_block_editor` when hooking into the `current_screen` action will always result in `false`, even when the block editor is being loaded. This is because `is_block_editor` is not set to `true` until `edit-form-blocks.php` is included.

This change adds logic to `WP_Screen` to ensure the accuracy of `is_block_editor` on block editor pages earlier in the load process.

While edit screens will now be accurate 100% of the time from `current_screen` on, there are still a few edge cases where `is_block_editor` could contain an incorrect value when creating a new post.

Because a `WP_Post` object is a required parameter for the `replace_editor` filter and `use_block_editor_for_post()` function, `WP_Screen` will fall back to the value returned by `use_block_editor_for_post_type()` for the post being created. To eliminate these edge cases, the `use_block_editor_for_post_type` filter can be used to return the appropriate boolean value to indicate support.

Props Chouby, desrosj, aduth, johnbillion.
Fixes #46195.

git-svn-id: https://develop.svn.wordpress.org/trunk@45224 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-advanced.php

@@ -18,6 +18,10 @@ if ( ! defined( 'ABSPATH' ) ) {
  */
 global $post_type, $post_type_object, $post;
 
+// Flag that we're not loading the block editor.
+$current_screen = get_current_screen();
+$current_screen->is_block_editor( false );
+
 if ( is_multisite() ) {
 	add_action( 'admin_footer', '_admin_notice_post_locked' );
 } else {