:label supports Civet reserved words (but not JS reserved words)

[edemaine]

Fixes #1638

• Allow reserved words in :label
  • JavaScript reserved words will fail on output
  • Civet reserved words like loop and and are useful
  • :void remains untouched for possible return type annotation...
• break :label can also use a reserved word, with the same caveats
• break loop also works like break :loop, because it's "natural" meaning while (true) break isn't useful.

It's debatable to me whether :void should be allowed as a return-type annotation. It's extremely fragile; for example, :number doesn't work as a return-type annotation at the beginning of a line (it's treated as a label). UPDATE: But I guess :void is common enough to be worth keeping.

We could allow JavaScript reserved words in :label (possibly excepting :void) by transpiling them to _label: (though we need to avoid conflicts here, and can't exactly use refs). I'm not sure how useful this would be. But it might be nice e.g. to write

:while while cond()
  ...
  break :while

[STRd6]

Thanks!