Add ability to double-enter to create a new list item

[aurooba]

Right now the only way to add another list item is to either use the Add button in the list-item toolbar or use the block inserter. It would be seriously handy if you could just press Enter twice while in a list item, to add a new list item underneath and switch focus to the first paragraph in it to start typing.

[aurooba]

Related #6

[aurooba]

Upon some reflection, I'm not sure this is the right method. Using a an enter transform with maybe the characters >> might be more in line with what the Block Editor API already provides.