feat/bug(commonmark] Block elements (including indented code blocks) under list items

[yellis]

Is your feature request related to a problem? Please describe.
Cannot make block elements under list items (works in old editor)

Example scenarios that aren't working:

(1) (from https://meta.stackexchange.com/a/380355/51) Put in this text

You can solve this in the following ways:

1.     println("Hello, world!")

2.     System.out.println("Hello, world!")

switch to RT and it shows code blocks on new lines after the list items. Switch back and it stays that way in markdown.

We could say that if you want a code block inside a list item, you need to use backticks (the commonmark spec does not require indented code blocks in list items, though it is ambiguous). Or we could allow it (preferred, to be same as old editor). But either way, round-trip shouldn't change anything.

(2) From https://meta.stackexchange.com/a/380344/51, notes that the old editor allowed block elements (code block / heading / blockquote / table) inside list items, and the new one pushes them to new lines (as seen in the previous issue). Indeed, the menu items for block elements are turned of in RT mode when the cursor is inside a list item. Would be good if we can add this capability back, for parity. See that post for screenshots.