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.

[Witoso]

Somehow it duplicated itself: #14634