Org writer: support starting number cookies

[soficshift]

This complements #7806 by supporting writing Org ordered lists that start at a specific number.

This commit also slightly changes another thing: before, the org writer indented items to match the markers length like so

9.  nine
10. ten

I was confused about what it should do when a counter cookie was present (should its length be counted as a marker, or ignored as part of the text like checkboxes?). But then I saw that org mode in emacs did not like this indentation anyway, because when you trigger the automatic formatting (C-c C-c above the list) emacs removes the extra spaces. So after this commit, the aligment is removed and the last list is written as

9. [@9] nine
10. ten

Is this ok?

Also, is one test enough? I did wrote another one with an empty item that would not pass for mysterious reasons, only to discover that the issue was not related to this (see #7810) 🙃

[soficshift]

Oops, I was not aware of those test/writer.org tests. I think they are not run when I filter -p Org, tomorrow I will fix them

[jgm]

Looks good , thanks!