async fs api support

[milahu]

"fs in the browser" is provided by multiple libraries

• https://github.com/jvilk/BrowserFS
• https://github.com/isomorphic-git/lightning-fs
• https://github.com/filerjs/filer

persistence usually via IndexedDB

they all implement the standard node fs api, but only the async functions

fs.readDir
fs.readFile

etc

feature request:
bash-emulator should support the async fs api by default

other tools compatible with the async fs api

• https://github.com/isomorphic-git/isomorphic-git