Add support for `withFileTypes` in `fs.readdir` calls.

[LeifAndersen]

As I understand it, node v10 added withFileTypes for the options field in fs.readdir and fs.readdirSync. This returns file metadata along with file types.

https://nodejs.org/api/fs.html#fs_fs_readdir_path_options_callback

It seems like BrowserFS currently doesn't have this:

> var fs = BFSRequire("fs")
> configure({fs: "LocalStorage"}, () => {console.log("Done")})
Done
> fs.readdirSync("/")
["test.txt"]
> fs.readdirSync("/", {withFileTypes: true})
["test.txt"]

[james-pre]

Please use zen-fs/core#19