Add editor_stylesheets filter to get_editor_stylesheets().

props danielbachhuber, MikeHansenMe.
fixes #31672.

git-svn-id: https://develop.svn.wordpress.org/trunk@32928 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1499,7 +1499,14 @@ function get_editor_stylesheets() {
 			}
 		}
 	}
-	return $stylesheets;
+	/**
+	 * Filter the array of stylesheets applied to the editor.
+	 *
+	 * @since 4.3.0
+	 *
+	 * @param array $stylesheets Array of stylesheets to be applied to the editor.
+	 */
+	return apply_filters( 'editor_stylesheets', $stylesheets );
 }
 
 /**