Add WYSIWYG/CKEditor 5 button for inserting spoiler

Going to start on this.

From ✨ Add a CKEditor5 plugin for Drupal 9.3+, Drupal 10 Needs work :

Official Drupal documentation for building a plugin:
https://www.drupal.org/docs/drupal-apis/ckeditor-5-api/overview →

Official Drupal docs for rewriting a plugin:
https://www.drupal.org/docs/core-modules-and-themes/core-modules/ckedito... →

Official CKEditor documentation for rewriting a plugin:
https://ckeditor.com/docs/ckeditor5/latest/installation/getting-started/...

Example module:
https://www.drupal.org/project/ckeditor5_dev →

Blog posts on the topic:
https://evolvingweb.com/blog/porting-ckeditor-5-plugin-drupal-10
https://tech.axelerant.com/create-and-integrate-ckeditor-5-plugin-with-d...

WIP bits for the plugin.

The plugin shows up, can be added, but it doesn't work yet. And #3388923 hasn't been done yet.

Additional suggestion from Wim Leers:
* https://ckeditor.com/docs/ckeditor5/latest/framework/guides/plugins/crea...
* https://ckeditor.com/docs/ckeditor5/latest/framework/plugins/package-gen...
* https://ckeditor.com/docs/ckeditor5/latest/tutorials/abbreviation-plugin...

This corrects the toolbar icon.

Trying to use the HTML tag "drupal-spoiler" in CKEditor.

Correction - drupalspoiler not drupal-spoiler.

Some food for thought -- definitely not perfect, but it "techincally" seems to work :-).

The main changes were to the `spoilerediting.js` and `insertspoilercommand.js` files. I'm registering three new models with the schema: `drupalspoiler`, `drupalspoiler_warning`, and `drupalspoiler_content`. The upcast / downcast transform these models back and forth from

<span class="spoiler">
  <span class="spoiler-warning">Arbitrary Text Here</span>
  <span class="spoiler-content">Arbitrary Text Here</span>
</span>

Thank you ever so much!

Just to explain my goal - I basically want this to be like a B, STRONG, I or EM tag that just wraps the text so that it can be inline; it'll use CSS to modify how it looks on the page, but at the ckeditor level there shouldn't be any additional customization.

So the patch is a huge improvement in that it will make the plugin actually work, but I just want it to be like this:

Original text:
<p>Something really funny.</p>

I want to be able to select the words "really funny", click the Spoiler toolbar button and have it wrap the text in the spoiler logic.

Output:
<p>Something <span class="spoiler">really funny</span>.</p>

Model:
<p>Something <drupalspoiler>really funny</drupalspoiler.</p>

If it'd be simpler to do, the extra drupalspoiler layer could be ditched and just use <span class="spoiler>really funny</span> on both ends.

In Drupal Slack, @Luke.Leber recommended starting from https://ckeditor.com/docs/ckeditor5/latest/tutorials/widgets/implementin.... Seconding that.

And in addition to that: I'd also look at/compare with the language.TextPartLanguage plugin’s code, because:

ckeditor5_language:
  ckeditor5:
    plugins: [language.TextPartLanguage]
  drupal:
…
    elements:
      - <span lang dir>

👆 that can take the selected text and wrap it in <span lang>, which is close to what you want!

P.S.: It is conceivable somebody has been using <span class="spoiler"> already. For that reason, if you were to get rid of the <drupalspoiler> representation, I'd not go for <span class="spoiler"> but <span spoiler>, to avoid that potential conflict. This module has a corresponding filter anyway, so it'd be trivial to transform it at that point into whatever markup you like :) However, I suspect you're trying to get rid of the filter, and so immediately dataDowncasting to <span class="spoiler"> seems fine too, and would allow you to just close ✨ Provide CKEditor 5-compatible filter Active 👍