Remove polyglot mode #850

This will invalidate #731 #702 and #132 and address part of #800

The gist of the plan is

• polyglot mode is cool! but it's not the main usecase of documentation and has some real drawbacks
  • the code it uses to parse comments, get-comments, is unmaintained and has a critical bug
  • it interferes with the logic of our file extension getting and makes that process confusing

I'd like to still support C++ and other languages in the future! But I'm much happier doing so by separating the extraction & input phases to the degree that documentation.js can read the output of another module that extracts JSDoc comments from C++ code, rather than having CPP support in it.