Use CKEditor 5's native <ol start> support (and also support <ol reversed>)

I was checking this out with @hooroomoo and ran into a test failure that (at least at the moment) cant tell if it's a problem with the test, this implementation, or a new edge case.
SmartDefaultSettingsTest is failing on the filter_only__filter_html can be switched to CKEditor 5 without problems (3 upgrade messages) iteration. With the error The following tags/attributes are not allowed in the updated text format:<ol type="1 A I"> This failure is when $unsupported_tags_attributes isn't empty

Here's how $unsupported_tags_attributes made:

 $allowed_tags = HTMLRestrictions::fromTextFormat($text_format);
      $enabled_plugins = array_keys($this->manager->getEnabledDefinitions($updated_text_editor));
      $updated_allowed_tags = new HTMLRestrictions($this->manager->getProvidedElements($enabled_plugins, $updated_text_editor));
      $unsupported_tags_attributes = $allowed_tags->diff($updated_allowed_tags);

So it's the diff of $allowed_tags against $updated_allowed_tags. $updated_allowed_tags includes <ol type> (now offered in the), which implicitly allows the <code><ol type="1 A I"> that is deemed unsupported. diff() is intentionally written this way - if the caller is more restrictive it is returned as part of the diff.

So two questions come to mind:

1. Is adding the type attribute in this issue's scope?
2. If yes, should the test be refactored to account for situations where SmartDefaultSettings grants any-attribute-value in CKeditor5 when it's predecessor was resticted to specific ones?

These may answer themselves once I'm away from the IDE for a moment, but now it's documented.

Just a few nits that can be easily taken care of as part of the reroll.

1. +++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditor5Test.php
   @@ -442,4 +442,101 @@ public function testEmphasis() {
   +    $ordered_list_html = '<ol>
   +    <li>
   +        apple
   +    </li>
   +    <li>
   +        banana
   +    </li>
   +    <li>
   +        cantaloupe
   +    </li>
   +</ol>';
   

   Lets either get rid of the line breaks or make this HEREDOC. (No line breaks is fine, though, it's clear what is being added).

2. +++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditor5Test.php
   @@ -442,4 +442,101 @@ public function testEmphasis() {
   +    $numbered_list_dropdown = '.ck-splitbutton__arrow';
   

   NIT: This, and $reversed_order_button and $start_index_element should get renamed to make it clear they are selectors, not the actual element (such as what might be returned by $page->find())

Self-RTBC'ing because super minor changes made

Committed e935c43 and pushed to 10.0.x. Thanks!

Leaving open for 9.4.x and 9.3.x backport.

Backported to 9.4.x + 9.3.x