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/block-patterns/query-grid-posts.php

@@ -14,7 +14,7 @@ return array(
 					<!-- wp:post-template -->
 					<!-- wp:group {"style":{"spacing":{"padding":{"top":"30px","right":"30px","bottom":"30px","left":"30px"}}},"layout":{"inherit":false}} -->
 					<div class="wp-block-group" style="padding-top:30px;padding-right:30px;padding-bottom:30px;padding-left:30px"><!-- wp:post-title {"isLink":true} /-->
-					<!-- wp:post-excerpt {"wordCount":20} /-->
+					<!-- wp:post-excerpt /-->
 					<!-- wp:post-date /--></div>
 					<!-- /wp:group -->
 					<!-- /wp:post-template -->