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.

git-svn-id: https://develop.svn.wordpress.org/trunk@55179 602fd350-edb4-49c9-b593-d223f7449a82

src/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',
 	);