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.

Built from https://develop.svn.wordpress.org/trunk@51443


git-svn-id: http://core.svn.wordpress.org/trunk@51054 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme-json.php

@@ -1178,7 +1178,7 @@ class WP_Theme_JSON {
 				$theme_settings['settings']['spacing'] = array();
 			}
 			$theme_settings['settings']['spacing']['units'] = ( true === $settings['enableCustomUnits'] ) ?
-				array( 'px', 'em', 'rem', 'vh', 'vw' ) :
+				array( 'px', 'em', 'rem', 'vh', 'vw', '%' ) :
 				$settings['enableCustomUnits'];
 		}