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.

git-svn-id: https://develop.svn.wordpress.org/trunk@56025 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/common.css

@@ -573,7 +573,8 @@ code {
 	margin: 10px 20px 0 2px;
 }
 
-.wrap.block-editor-no-js {
+.wrap.block-editor-no-js,
+.wrap.site-editor-no-js {
 	padding-left: 20px;
 }