Numbered list

[Jjagg]

It'd be nice to have a bidirectional iterator that can iterate paragraphs of both a BoustroDocument and a DocumentController. A copy of the iterator can then be passed when building a paragraph.

I think that'd be the cleanest way to add numbered list support. This system would mean every numbered list paragraph would have to count all items before it (O(N^2)). Though that's fine considering the low number of lines we're dealing with.

Alternatively we could have some state object that we pass through to every line for them to track state during the build step. This is less flexible, but more efficient.

Either way we really don't want to bake the line counting into boustro's core systems. It should be handled by the component itself.