Collab: Concurrent editing of List block with nested items

Scoped out of #112

Truly concurrent editing of nested lists is [not supported yet](https://github.com/Automattic/isolated-block-editor/blob/3e072e03e21248b0a17d4d184776c8bf30886f69/src/components/collaborative-editing/use-yjs/algorithms/__tests__/rich-text.js#L178-L189). I can think of a way to make it work in _most_ cases, but any more than that I think it would require better support at the Yjs (`Y.XmlText`) level.

Start by reading the linked rich-text.js test around lines 178-189 and the surrounding collaborative-editing Yjs algorithms. Reproduce concurrent edits to a List block with nested items, then determine the supported cases and remaining Y.XmlText limitations. Done means the intended concurrent-editing cases are covered without regressing existing behavior.

Written by the indexing model from the issue text.