forward-delete in list items

[bantic]

This fixes #118

Adds test cases for:

• forward-delete when at end of a (non-empty) list item and:
  • there is no markerable section immediately following it (i.e., end of document or card section following)
  • there is a list item following it
  • there is a markup section following it
• forward-delete when cursor is in empty list item and:
  • there is no markerable section immediately following it
  • there is a list item following it
  • there is a markup section following it