Experiment: port vim.js to use our filesystem

[VIM.js](https://github.com/coolwanglu/vim.js) is really neat. It's the full VIM compiled to JS via ecmascripten. It would be cool to see if we can add it to our platform (e.g., `/editor/vim/...` or something).

The file handling is mostly contained in https://github.com/coolwanglu/vim.js/blob/master/web/vim_lib.js (look for `FS.`). Most of it assumes you call it synchronous, but I think we could swap async Filer versions of these functions without too much problem.