Add debugger code for node.js.

In preparation to support realgud debugger handling for node.js. I tested this with `make recipes/realgud` and looked at the resulting tar file. 

Right now, the node.js code is in a branch on realgud. I believe this has to be the case before this change or a change like this (see issue #1602) or else melpa installs will fail when trying to require the new code that's not in the tar file. 

Thanks.