Block Editor: Backport fixes targetted for WordPress 5.8 RC4.

This includes:

 - Suggestion List: Check if a node exists to scroll into view.
 - Autocomplete: reset state for empty text.
 - Adds auxiliary class names for editor styles in the widgets editor.
 - Extract snackbars into a separate component.
 - Rich text: run input rules after composition end.
 - iframe: load inline styles.
 - Multi select: select all: restore ref callback.
 - Writing flow: allow select all from empty selection.
 - Post Excerpt: Fix excerpt_more filter conflict and remove wordCount attribute.
 - Add the percent unit to the default units in Core.

Props desrosj.
See #53397.


git-svn-id: https://develop.svn.wordpress.org/trunk@51443 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/theme/wpThemeJson.php

@@ -925,7 +925,7 @@ class Tests_Theme_wpThemeJson extends WP_UnitTestCase {
 					),
 				),
 				'spacing'    => array(
-					'units' => array( 'px', 'em', 'rem', 'vh', 'vw' ),
+					'units' => array( 'px', 'em', 'rem', 'vh', 'vw', '%' ),
 				),
 				'typography' => array(
 					'customFontSize'   => false,
@@ -1039,7 +1039,7 @@ class Tests_Theme_wpThemeJson extends WP_UnitTestCase {
 		$input = get_default_block_editor_settings();
 
 		$expected = array(
-			'units'         => array( 'px', 'em', 'rem', 'vh', 'vw' ),
+			'units'         => array( 'px', 'em', 'rem', 'vh', 'vw', '%' ),
 			'customPadding' => false,
 		);