extract fs dependency

part of #4

Suggesting to extract Node.js dependencies into separate files per module, in a ``nodejs`` subdirectory.

Here, functions with ``fs``-specific code are extracted into ``nodejs/fsReader.js``. Still need to pass the file descriptor ``fd`` through functions in ``pbfParser.js``, but now that could just as well be an ``ArrayBuffer``. Already adds ``nodejs/buffer.js`` because of a common dependency.