Add simple command-line code generator tool

This adds a command-line tool which can generate JS from Sapl files.

It outputs to stdout. It does not include a JS beautifier; there are plenty of tools for that (e.g. js-beautify, uglify-js).

I still want to add an option which reads the ABC file for dependencies and ensures the generated JS is self-contained. That can be done before or after this MR gets merged.