Blorb: Standardise chunks for JS and CSS

[curiousdannii]

I think it would be good to specify standard chunks for JS and CSS. Now that Lectrote is becoming more popular it would be nice to be able to distribute the CSS in one blorb file, and for that to also be loaded in Quixe and Parchment. (The DBug chunk also isn't mentioned in the spec - being an extensible format this isn't necessary, but it could be good to note anyway.)

It would also be worth considering whether there is any aspect of current GlkOte's output which you'd want to ever change. For example, I note that it currently outputs Style_header but the Glk stylesheets draft suggests Para_Header and Span_Bold. But this could probably be changed to use the current class name style for spans.