Editor: Separate content styles for iframe in __wp_get_iframed_editor_assets() and wp_default_styles().

This changeset:
* Removes the `'wp-block-editor'` and `'wp-block-library'` from the iframe's stylesheet collection (i.e. `_wp_get_iframed_editor_assets()`).
* Adds the new `'wp-block-editor-content'` for the separate stylesheet to the list of editor (`'wp-edit-blocks'`) dependencies.

Why?

These PHP changes are part of the initiative to:

* Separate the content styles contained in the block editor package into a separate stylesheet.
* Avoid loading all block editor styles into the iframe.

References:
* [https://github.com/WordPress/gutenberg/pull/44298 Gutenberg PR 44298].

Follow-up to [53160], [50761].

Props ellatrix, youknowriad.
Fixes #57550.
Built from https://develop.svn.wordpress.org/trunk@55179


git-svn-id: http://core.svn.wordpress.org/trunk@54712 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/block-editor.php

@@ -300,8 +300,6 @@ function _wp_get_iframed_editor_assets() {
 
 	$script_handles = array();
 	$style_handles  = array(
-		'wp-block-editor',
-		'wp-block-library',
 		'wp-edit-blocks',
 	);
 

wp-includes/script-loader.php

@@ -1609,6 +1609,12 @@ function wp_default_styles( $styles ) {
 		array()
 	);
 
+	$styles->add(
+		'wp-block-editor-content',
+		"/wp-includes/css/dist/block-editor/content$suffix.css",
+		array()
+	);
+
 	$wp_edit_blocks_dependencies = array(
 		'wp-components',
 		'wp-editor',
@@ -1617,6 +1623,7 @@ function wp_default_styles( $styles ) {
 		'wp-reset-editor-styles',
 		'wp-block-library',
 		'wp-reusable-blocks',
+		'wp-block-editor-content',
 	);
 
 	// Only load the default layout and margin styles for themes without theme.json file.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55178';
+$wp_version = '6.2-alpha-55179';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.