Add DocumentLists support behind a flag

[CKEditor5]

So, Document lists (ckeditor/ckeditor5#2973) are now available as an experimental feature.

It'd be cool if we could test this feature in GH Writer.

It's perhaps too early to turn it on by default so this could be hidden behind some flag.

Potential problems:

• Markdown support for the DocLists output. It's much more complex and there's a risk of often running into Markdown's inability to reflect the HTML outputted by DocLists.
• Stability of DocLists for now.

[mlewand]

@mateuszzagorski let's give it a try if it can be easily integrated. Let's work on this up to 2 mds. If it doesn't come easily we'll drop it for time being.

[mateuszzagorski]

As stated in the docs  - the DocumentList can not be loaded together with the 'List" plugin - so as of this moment we would have to turn the ToDoList plugin off, as they cannot work together in the current state.

Branch where you can test the DocumentList working can be found here: ghw/314-document-list-integration.

Short video showing how it looks like when used with the GitHub Writer:

https://user-images.githubusercontent.com/11699569/188606350-3e167250-dc17-4110-bf8b-4874110b04fe.mov

[mlewand]

Unfortunately despite the fact that conversion (to md) works fine, we need to drop this improvement for now because todo list is also essential feature of GH writer.

For now it seems that integrating todolists with document lists is a bigger task and we don't have the means to do it .

[Reinmar]

That's why I proposed:

It's perhaps too early to turn it on by default so this could be hidden behind some flag.

It can be implemented in a really simple way – by reading that flag from a local storage.

And since the goal here is to enable DocLists for the core CKE5 team, that's completely enough.