JS importer has been improved

[Edward K. Ream]

#1481 has been completed using a proper JS tokenizer. See the first comment for full details.

As noted in the first comment, imo it's best to import to a "live" @clean, provided that the original sources are under control of git or some other sccs.

The first comment also shows what imo is the clearest top-level node for reveal.js. It was fairly straightforward to find this, but it would take true AI to get it automatically.

Let me know if problems remain.

Edward