Add a 9P interface to our filesystem

Depending on the outcome of our research in #26, we're going to need a way to mount the IndexedDB web filesystem (probably [filer](https://github.com/filerjs/filer)) in the browser VM for our OS/shell. Just about every example I see is using the Plan9 filesystem (9P) implementation for Linux [v9fs](https://lwn.net/Articles/137439/).

The protocol is pretty slim, transport agnostic (we can easily do it over message passing between frames/windows/workers), and would map well onto our filesystem. There are also a number of [existing JavaScript implementations](http://9p.cat-v.org/implementations), including (and in order of how well maintained they appear to be):

* [js9p](https://github.com/anantn/web9/tree/master/js9p) (MIT)
* [limbo-machine](https://code.google.com/archive/p/limbo-machine/wikis/JS.wiki) (LGPL)
* [arm-js](https://github.com/ozaki-r/arm-js) (MIT, LGPL)
* [jor1k](https://github.com/s-macke/jor1k) (BSD)
* [v86](https://github.com/copy/v86) (BSD)

The goal would be to allow our VM (probably v86) to mount our filesystem over 9P into `/home` or the like, and work with the files and dirs as if they were in the VM.