Editor: Add no-js fallback for site editor.

Add a fallback condition with heading and error notice to handle a no JavaScript state for the site editor, comparable to what already exists in the post editor. 

Props afercia, joedolson, fencermonir, zebaafiashama, alexstine, rudlinkon.
Fixes #56228.
Built from https://develop.svn.wordpress.org/trunk@56025


git-svn-id: http://core.svn.wordpress.org/trunk@55537 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/common-rtl.css

@@ -574,7 +574,8 @@ code {
 	margin: 10px 2px 0 20px;
 }
 
-.wrap.block-editor-no-js {
+.wrap.block-editor-no-js,
+.wrap.site-editor-no-js {
 	padding-right: 20px;
 }