Make the npm package more useful

[rocketnia]

Currently, the npm package can't be used for much. It can build the demos and run tests, but that's not important for it to do.

There are basically four features in this repo: The reader, Penknife for JS, raw Staccato, and Staccato mini. (There's also the module system and era-tacit.js, but I don't think these ever got to a practical state.) What's most important is probably to allow libraries and applications to be written in these languages and to allow the reader to be used to make new languages.

Let's enable a workflow where libraries written in these languages offer commands to copy their files into a given directory.

The era-platform package can offer commands to copy in files that constitute the essential core libraries. It can also offer commands to run code, to run a REPL (if the language supports it), to compile code to JS (if the language supports it), or to prepare a browser demo based on some given files.