List v2 + RichText: allow trailing non editable content

[ellatrix]

What problem does this address?

When nesting another list in a list item, this list is inserted inside the list item. Ideally this list item text should also be editable. To work around this, we wrapped the text in an editable div, but ideally we shouldn't have to do that.

The markup should look as follows:

<li contenteditable="true">
  some text
  <ul contenteditable="false">...</ul>
</li>

The current markup is this:

<li>
  <div contenteditable="true">
    some text
  </div>
  <ul>...</ul>
</li>

What is your proposed solution?

It will require modifying RichText so we can pass a before/after prop containing the element to render before or after the text.