Document lists should have a “simple list” config option

[Witoso]

As we decided to make the Document list the only plugin that introduces the lists, we need to add a config option that still allows creating simple lists.

Why?

Integrators may want to allow simpler content editing, and block creation of blocks in the lists. We have a similar use case in our comments' system. You rarely may want to have a complex markup in small editors. etc.

Scope

• This option should be opt-in. By default, lists will allow blocks.
• After setting this option
  • I can only insert list items. Simplified data output <ul><li>a</li><li>b</li></ul> (for ol as well).
  • Typing actions that we have in the document lists (e.g., enter+backspace creates a block) should work differently (as current lists).
• Documentation.
  • Simple list config in the feature.
  • Release and update guide information.

[Witoso]

Refinement notes:

• When there's an option in the config, don't register the command that merges blocks, etc.
• Schema needs to change (e.g., no heading in the list item).
• Postfixers: all id's in list items need to be unique.

[niegowski]

Should we deny multiple types of nested lists in a single list item? Shall simple list allow such structure or should it not (in legacy lists it was not possible):

<ul>
    <li>
        a
        <ol>
            <li>
                b
            </li>
        </ol>
        <ul>
            <li>
                c
            </li>
        </ul>
    </li>
</ul>

[Witoso]

I don't see a reason why not. I see it as a good enhancement for the user that they can have a bulleted list and inside it have a numbered one. It still matches my definition of simple.

@Reinmar do you remember why Lists v1 didn't have it? Or was it only scope cutting?