Hook exit() in emscripted languages.

[max99x]

Emscripted languages can call exit() which will raise a JS error and might render the interpreter non-functional. Currently we just print the exception as an internal error and continue as if nothing happened. Instead we should provide a way to hook into this call (e.g. via an exit event) so that clients can properly respond to exiting the interpreter. E.g. for repl.it that might mean printing an exit message and not restarting the prompt, or returning to the languages selector.