include syntax-highlighted code in standalone web pages

just like "Save as Applet" has

I decided to take this on partly because:

 * the syntax-highlighting code was removed from the headless branch a while back; I'm the best-equipped person to bring it back...
 * ...because compiler changes means the needed information for colorization isn't all in Tokenizer anymore it's split between Tokenizer and Namer
 * and that will need some sorting out

and partly because I think including the code emphasizes that Tortoise is actually running actual NetLogo code, that we didn't just build models that _look_ like the NetLogo originals

this involves work in both the NetLogo repo (headless branch) and the Teletortoise repo.