Editor: Add is block theme setting to get_block_editor_settings().

The block editor's iframe needs to know if the theme is a block theme. This changeset adds a setting to `get_block_editor_settings()`.

Reference:
* WordPress/gutenberg#46212

Props ellatrix, alexstine, costdev, dgwyer, glendaviesnz, scruffian.
Fixes #57549.

git-svn-id: https://develop.svn.wordpress.org/trunk@55147 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-editor.php

@@ -495,6 +495,7 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 	}
 
 	$editor_settings['__unstableResolvedAssets']         = _wp_get_iframed_editor_assets();
+	$editor_settings['__unstableIsBlockBasedTheme']      = wp_is_block_theme();
 	$editor_settings['localAutosaveInterval']            = 15;
 	$editor_settings['disableLayoutStyles']              = current_theme_supports( 'disable-layout-styles' );
 	$editor_settings['__experimentalDiscussionSettings'] = array(