implemented Node:insert before

Hopefully this is more useful than my previous contributions.

Do you think that it makes sense to make Node.childNodes readOnly? (cf. https://github.com/istr/lua-gumbo/commit/de76afd87c8ba593fc7fa753001425db50a7d6fd and https://github.com/istr/lua-gumbo/commit/084cf736a9568b7ec5a445c13859ffd79a25d45e)