Remove duplicate code.
Generic 'scoped_no_eols' implementation.

So that it can be reused for other state variables.
Use a value in local for simple_markup's mark.

Feels a little better to me. Although it isn't as stronly associated with the
rule.
Use a value in `local` for `still_in_block`.

Allows `paragraph_item` and `list_item` to be merged into a single rule.
Use a value in `local` for `context`.